TypeScript to ArkTS Cookbook

Welcome to the "TypeScript to ArkTS" cookbook. This document gives you short recipes to rewrite your standard TypeScript code to ArkTS. Although ArkTS is designed to be close to TypeScript, some limitations were added for the sake of performance. As a result, all TypeScript features can be divided into the following categories:

Fully supported features: The original code requires no modification at all. According to our measurements, projects that already follow the best TypeScript practices can keep 90% to 97% of their codebase intact.
Partially supported features: Some minor code refactoring is needed. Example: The keyword let must be used in place of var to declare variables. Your code will remain a valid TypeScript code after rewriting by our recipes.
Unsupported features: A greater code refactoring effort is required. Example: The type any is unsupported, and you need to introduce explicit typing to your code everywhere any is used.

The document is built on the feature-by-feature basis, and if you do not find a specific feature, then you can safely consider it fully supported. Otherwise, a recipe will give you a suggestion on how to rewrite your code and work around an unsupported case.

Recipe Explained

The original TypeScript code contains the keyword var:

function addTen(x: number): number {
  var ten = 10
  return x + ten
}

The code must be rewritten as follows:

// Important! This is still valid TypeScript code.
function addTen(x: number): number {
  let ten = 10
  return x + ten
}

Severity Levels

Each recipe is marked with the the severity level. Supported values:

Severity: error: The recipe should be followed, otherwise the program will fail to compile.
Severity: warning: It is highly recommended to follow the recipe. Although violating the recipe does not currently affect the compilation, in future versions, it will cause compilation to fail.

Status of Unsupported Features

Currently, unsupported features are mainly either of the following:

Features relate to dynamic typing that degrades runtime performance
Features that require extra support in the compiler, thereby degrading project build time

However, the ArkTS team reserves the right to reconsider the list and shrink it in the future releases based on the feedback from developers and real-world data experiments.

Recipes Summarized

This document provides an informal summary of TypeScript features that ArkTS either can support with limitations, or cannot support. See Recipes for the full list with more detailed code examples and workaround suggestions.

Static Typing Is Enforced

ArkTS was designed with the following goals in mind:

ArkTS programs must be easy for developers to read and understand because the code is read more often than written.
ArkTS must execute fast and consume as little power as possible because it is particularly critical on mobile devices that ArkTS targets.

One of the most important features of ArkTS that helps achieving both goals is static typing. If a program is statically typed (all types are known at compile time), it is much easier to understand what data structures are used in the code. Since all types are known before the program actually runs, the compiler can verify code correctness, thereby eliminating many runtime type checks and improving performance.

Therefore, the usage of the type any in ArkTS is prohibited.

Example

// Not supported:
let res: any = some_api_function('hello', 'world')
// What is `res`? A numeric error code, a string, or an object?
// How should we work with it?
// Supported:
class CallResult {
  public succeeded(): boolean { ... }
  public errorMessage(): string { ... }
}

let res: CallResult = some_api_function('hello', 'world')
if (!res.succeeded()) {
  console.log('Call failed: ' + res.errorMessage())
}

According to our measurements, any is already not welcome in TypeScript. It is used in approximately 1% of TypeScript codebases. Moreover, today's code linters (for example, ESLint) include a set of rules that prohibit the usage of any. Prohibiting any results in a strong positive impact on performance at the cost of low-effort code refactoring.

Changing Object Layout in Runtime Is Prohibited

To achieve maximum performance benefits, ArkTS requires the layout of objects to remain unchanged during program execution. In other words, it is prohibited to:

Add new properties or methods to objects.
Delete existing properties or methods from objects.
Assign values of arbitrary types to object properties.

It is noteworthy that many such operations are already prohibited by the TypeScript compiler. However, it can still be "tricked", for example, by as any casting that ArkTS does not support (see the detailed example below).

Example

class Point {
  public x: number = 0
  public y: number = 0

  constructor(x: number, y: number) {
    this.x = x
    this.y = y
  }
}

// It is impossible to delete a property from the object. It is guaranteed that all Point objects have the property x.
let p1 = new Point(1.0, 1.0)
delete p1.x           // Compile-time error in TypeScript and ArkTS
delete (p1 as any).x  // OK in TypeScript; compile-time error in ArkTS

// Class Point does not define any property named `z`, and it is impossible to add it while the program runs.
let p2 = new Point(2.0, 2.0)
p2.z = 'Label';           // Compile-time error in TypeScript and ArkTS
(p2 as any).z = 'Label'   // OK in TypeScript; compile-time error in ArkTS

// It is guaranteed that all Point objects have only properties x and y, it is impossible to generate some arbitrary identifier and use it as a new property:
let p3 = new Point(3.0, 3.0)
let prop = Symbol();     // OK in TypeScript; compile-time error in ArkTS
(p3 as any)[prop] = p3.x // OK in TypeScript; compile-time error in ArkTS
p3[prop] = p3.x          // Compile-time error in TypeScript and ArkTS

// It is guaranteed that all Point objects have properties x and y of type number, so assigning a value of any other type is impossible:
let p4 = new Point(4.0, 4.0)
p4.x = 'Hello!';         // Compile-time error in TypeScript and ArkTS
(p4 as any).x = 'Hello!' // OK in TypeScript; compile-time error in ArkTS

// Usage of Point objects which is compliant with the class definition:
function distance(p1: Point, p2: Point): number {
  return Math.sqrt(
    (p2.x - p1.x) * (p2.x - p1.x) + (p2.y - p1.y) * (p2.y - p1.y)
  )
}
let p5 = new Point(5.0, 5.0)
let p6 = new Point(6.0, 6.0)
console.log('Distance between p5 and p6: ' + distance(p5, p6))

Unpredictable changing of object layout contradicts both good readability and good performance of code. Indeed, having class definition at one place and modifying actual object layout elsewhere is confusing and error-prone from the developer's point of view. It opposes the idea of static typing and requires extra runtime support that causes undesired execution overhead.

According to our observations and experiments, this feature is already not welcome in TypeScript. It is used in a marginal number of real-world projects, and state-of-the-art linters have rules to prohibit its usage.

Prohibiting runtime changes to object layouts results in a strong positive impact on performance at the cost of low-effort refactoring.

Semantics of Operators Is Restricted

To achieve better performance and encourage developers to write clearer code, ArkTS restricts the semantics of some operators. An example is given below, and the full list of restrictions is outlined in Recipes.

Example

// Unary `+` is defined only for numbers, but not for strings:
let t = +42; // OK
let s = +'42'; // Compile-time error

Loading language operators with extra semantics complicates the language specification, and forces developers to remember all possible corner cases with appropriate handling rules. Besides, in certain cases it causes some undesired runtime overhead.

According to our observations and experiments, this feature is not popular in TypeScript. It is used in less than 1% of real-world codebases, and such cases are easy to refactor. Restricting the operator semantics results in a clearer and more performant at the cost of low-effort changes in code.

Structural Typing Is Not Supported (Yet)

Assuming that two unrelated classes T and U have the same public API:

class T {
  public name: string = ''

  public greet(): void {
    console.log('Hello, ' + this.name)
  }
}

class U {
  public name: string = ''

  public greet(): void {
    console.log('Greetings, ' + this.name)
  }
}

Can we assign a value of T to a variable of U?

let u: U = new T() // Is this allowed?

Can we pass a value of T to a function that accepts a parameter of U?

function greeter(u: U) {
  console.log('To ' + u.name)
  u.greet()
}

let t: T = new T()
greeter(t) // Is this allowed?

In other words, which approach will we take:

T and U are not related by inheritance or any common interface, but they are "somewhat equivalent" since they have the same public API, and so the answer to both questions above is "yes".
T and U are not related by inheritance or any common interface, and always must be considered as totally different types, and so the answer to both questions above is "no".

The languages that take the first approach are said to support structural typing, whereas the languages that take the second approach do not support it. Currently, TypeScript supports structural typing, and ArkTS does not.

It is debatable whether structural typing helps to produce code that is clearer and more understandable, and both pro and contra arguments can be found. Why not just support it then?

The answer is that supporting structural typing is a major feature that needs a lot of considerations and careful implementation in language specification, compiler, and runtime. More importantly, in case if ArkTS, which enforces static typing (see above), runtime support for structural typing implies performance overhead. So since functionally correct and performant implementation requires taking that many aspects into account, the support to this feature is postponed.

The ArkTS team is ready to reconsider based on real-world scenarios and feedback. More cases and suggested workarounds can be found in Recipes.

Recipes

Recipe: Objects with Property Names That Are Not Identifiers Are Not Supported

Rule: arkts-identifiers-as-prop-names

Severity: error

ArkTS does not support objects with name properties that are numbers or strings. Use classes to access data by property names. Use arrays to access data by numeric indices.

TypeScript

var x = { 'name': 'x', 2: '3' }

console.log(x['name'])
console.log(x[2])

ArkTS

class X {
  public name: string = ''
}
let x: X = { name: 'x' }
console.log(x.name)

let y = ['a', 'b', 'c']
console.log(y[2])

// If you still need a container to store keys of different types, use Map<Object, some_type>.
let z = new Map<Object, string>()
z.set('name', '1')
z.set(2, '2')
console.log(z.get('name'))
console.log(z.get(2))

See also

Recipe: Symbol() Is Not Supported
Recipe: Indexed Access Is Not Supported for Fields
Recipe: delete Operator Is Not Supported
Recipe: typeof Operator Is Allowed Only in Expression Contexts
Recipe: in Operator Is Not Supported
Recipe: Usage of Standard Libraries Is Restricted

Recipe: `Symbol()` Is Not Supported

Rule: arkts-no-symbol

Severity: error

TypeScript uses the Symbol() API among other things to generate unique property names at runtime. ArkTS does not support the Symbol() API because its most popular use cases make no sense in the statically typed environment. In particular, the object layout is defined at compile time and cannot be changed at runtime.

Only Symbol.iterator is supported.

See also

Recipe: Objects with Property Names That Are Not Identifiers Are Not Supported
Recipe: Indexed Access Is Not Supported for Fields
Recipe: delete Operator Is Not Supported
Recipe: typeof Operator Is Allowed Only in Expression Contexts
Recipe: in Operator Is Not Supported
Recipe: Usage of Standard Libraries Is Restricted

Recipe: Private `#` Identifiers Are Not Supported

Rule: arkts-no-private-identifiers

Severity: error

ArkTS does not use private identifiers starting with the symbol #. Use the keyword private instead.

TypeScript

class C {
  #foo: number = 42
}

ArkTS

class C {
  private foo: number = 42
}

Recipe: Use Unique Names for Types and Namespaces

Rule: arkts-unique-names

Severity: error

Names for all types (classes, interfaces, and enums) and namespaces must be unique and distinct from other names such as, variable names, and function names.

TypeScript

let X: string
type X = number[] // Type alias with the same name as the variable

ArkTS

let X: string
type T = number[] // X is not allowed here to avoid name collisions.

Recipe: Use `let` Instead of `var`

Rule: arkts-no-var

Severity: error

ArkTS does not support var. Use let instead.

TypeScript

function f(shouldInitialize: boolean) {
  if (shouldInitialize) {
     var x = 'b'
  }
  return x
}

console.log(f(true))  // b
console.log(f(false)) // undefined

let upper_let = 0
{
  var scoped_var = 0
  let scoped_let = 0
  upper_let = 5
}
scoped_var = 5 // Visible
scoped_let = 5 // Compile-time error

ArkTS

function f(shouldInitialize: boolean): string {
  let x: string = 'a';
  if (shouldInitialize) {
    x = 'b';
  }
  return x;
}

console.log(f(true));  // b
console.log(f(false)); // a

let upper_let = 0
let scoped_var = 0
{
  let scoped_let = 0
  upper_let = 5
}
scoped_var = 5
scoped_let = 5 // Compile-time error

Recipe: Use Explicit Types Instead of `any` or `unknown`

Rule: arkts-no-any-unknown

Severity: error

ArkTS does not support the types any and unknown. Specify types explicitly.

TypeScript

let value1: any
value1 = true
value1 = 42

let value2: unknown
value2 = true
value2 = 42

ArkTS

let value_b: boolean = true // OR: let value_b = true
let value_n: number = 42 // OR: let value_n = 42
let value_o1: Object = true
let value_o2: Object = 42

See also

Recipe: Strict Type Checking Is Enforced

Recipe: Use `class` Instead of a Type with a Call Signature

Rule: arkts-no-call-signatures

Severity: error

ArkTS does not support call signatures in object types. Use class instead.

TypeScript

type DescribableFunction = {
  description: string
  (someArg: string): string // call signature
}

function doSomething(fn: DescribableFunction): void {
  console.log(fn.description + ' returned ' + fn(6))
}

ArkTS

class DescribableFunction {
  description: string
  public invoke(someArg: string): string {
    return someArg
  }
  constructor() {
    this.description = 'desc'
  }
}

function doSomething(fn: DescribableFunction): void {
  console.log(fn.description + ' returned ' + fn.invoke(6))
}

doSomething(new DescribableFunction())

See also

Recipe: Use class Instead of a Type with a Constructor Signature

Recipe: Use `class` Instead of a Type with a Constructor Signature

Rule: arkts-no-ctor-signatures-type

Severity: error

ArkTS does not support constructor signatures in object types. Use class instead.

TypeScript

class SomeObject {}

type SomeConstructor = {
  new (s: string): SomeObject
}

function fn(ctor: SomeConstructor) {
  return new ctor('hello')
}

ArkTS

class SomeObject {
  public f: string
  constructor (s: string) {
    this.f = s
  }
}

function fn(s: string): SomeObject {
  return new SomeObject(s)
}

See also

Recipe: Use class Instead of a Type with a Call Signature

Recipe: Only One Static Block Is Supported

Rule: arkts-no-multiple-static-blocks

Severity: error

ArkTS does not allow several static blocks for class initialization. Combine static block statements into one static block.

TypeScript

class C {
  static s: string

  static {
    C.s = 'aa'
  }
  static {
    C.s = C.s + 'bb'
  }
}

ArkTS

class C {
  static s: string

  static {
    C.s = 'aa'
    C.s = C.s + 'bb'
  }
}

NOTE

Currently, the static block syntax is not supported. This rule must be followed in .ets files when the syntax is supported.

Recipe: Indexed Signatures Are Not Supported

Rule: arkts-no-indexed-signatures

Severity: error

ArkTS does not allow indexed signatures. Use arrays instead.

TypeScript

// Interface with an indexed signature:
interface StringArray {
  [index: number]: string
}

function getStringArray(): StringArray {
  return ['a', 'b', 'c']
}

const myArray: StringArray = getStringArray()
const secondItem = myArray[1]

ArkTS

class X {
  public f: string[] = []
}

let myArray: X = new X()
const secondItem = myArray.f[1]

Recipe: Use Inheritance Instead of Intersection Types

Rule: arkts-no-intersection-types

Severity: error

Currently, ArkTS does not support intersection types. Use inheritance as a workaround.

TypeScript

interface Identity {
  id: number
  name: string
}

interface Contact {
  email: string
  phoneNumber: string
}

type Employee = Identity & Contact

ArkTS

interface Identity {
  id: number
  name: string
}

interface Contact {
  email: string
  phoneNumber: string
}

interface Employee extends Identity,  Contact {}

Recipe: Type Notation Using `this` Is Not Supported

Rule: arkts-no-typing-with-this

Severity: error

ArkTS does not support type notation using the this keyword. Use the explicit type instead.

TypeScript

interface ListItem {
  getHead(): this
}

class C {
  n: number = 0

  m(c: this) {
    // ...
  }
}

ArkTS

interface ListItem {
  getHead(): ListItem
}

class C {
  n: number = 0

  m(c: C) {
    // ...
  }
}

Recipe: Conditional Types Are Not Supported

Rule: arkts-no-conditional-types

Severity: error

ArkTS does not support conditional type aliases. Introduce a new type with constraints explicitly, or rewrite logic using Object. The keyword infer is not supported.

TypeScript

type X<T> = T extends number ? T: never
type Y<T> = T extends Array<infer Item> ? Item: never

ArkTS

// Provide explicit constraints within type alias.
type X1<T extends number> = T

// Rewrite with Object. Less type control requires more type checking for safety.
type X2<T> = Object

// Item has to be used as a generic parameter and needs to be properly instantiated.
type YI<Item, T extends Array<Item>> = Item

Recipe: Declaring Fields in `constructor` Is Not Supported

Rule: arkts-no-ctor-prop-decls

Severity: error

ArkTS does not support declaring class fields in constructor. Declare class fields inside the class declaration instead.

TypeScript

class Person {
  constructor(
    protected ssn: string,
    private firstName: string,
    private lastName: string
  ) {
    this.ssn = ssn
    this.firstName = firstName
    this.lastName = lastName
  }

  getFullName(): string {
    return this.firstName + ' ' + this.lastName
  }
}

ArkTS

class Person {
  protected ssn: string
  private firstName: string
  private lastName: string

  constructor(ssn: string, firstName: string, lastName: string) {
    this.ssn = ssn
    this.firstName = firstName
    this.lastName = lastName
  }

  getFullName(): string {
    return this.firstName + ' ' + this.lastName
  }
}

Recipe: Constructor Signatures Are Not Supported in Interfaces

Rule: arkts-no-ctor-signatures-iface

Severity: error

ArkTS does not support constructor signatures. Use methods instead.

TypeScript

interface I {
  new (s: string): I
}

function fn(i: I) {
  return new i('hello')
}

ArkTS

interface I {
  create(s: string): I
}

function fn(i: I) {
  return i.create('hello')
}

See also

Recipe: Use class Instead of a Type with a Constructor Signature

Recipe: Indexed Access Types Are Not Supported

Rule: arkts-no-aliases-by-index

Severity: error

ArkTS does not support indexed access types. Use the type name instead.

Recipe: Indexed Access Is Not Supported for Fields

Rule: arkts-no-props-by-index

Severity: error

ArkTS does not support dynamic field declaration and access. Declare all object fields immediately in the class. Access only those class fields that are either declared in the class, or accessible via inheritance. Accessing any other fields is prohibited, and causes compile-time errors.

To access a field, use the obj.field syntax. Indexed access (obj["field"]) is not supported. An exception is all typed arrays from the standard library (for example, Int32Array), which support access to their elements through the container[index] syntax. TypeScript

class Point {
  x: string = ''
  y: string = ''
}
let p: Point = {x: '1', y: '2'}
console.log(p['x'])

class Person {
  name: string = ''
  age: number = 0; // A semicolon is required here.
  [key: string]: string | number
}

let person: Person = {
  name: 'John',
  age: 30,
  email: '***@example.com',
  phoneNumber: '18*********',
}

ArkTS

class Point {
  x: string = ''
  y: string = ''
}
let p: Point = {x: '1', y: '2'}
console.log(p.x)

class Person {
  name: string
  age: number
  email: string
  phoneNumber: string

  constructor(name: string, age: number, email: string,
        phoneNumber: string) {
    this.name = name
    this.age = age
    this.email = email
    this.phoneNumber = phoneNumber
  }
}

let person = new Person('John', 30, '***@example.com', '18*********');
console.log(person['name']);     // Compile-time error
console.log(person.unknownProperty); // Compile-time error

let arr = new Int32Array(1);
arr[0];

Recipe: Structural Typing Is Not Supported

Rule: arkts-no-structural-typing

Severity: error

Currently, ArkTS does not support structural typing. This means that the compiler cannot compare public APIs of two types and decide whether they are identical. Use other mechanisms (inheritance, interfaces, or type aliases) instead.

TypeScript

interface I1 {
  f(): string
}

interface I2 { // I2 is structurally equivalent to I1.
  f(): string
}

class X {
  n: number = 0
  s: string = ''
}

class Y { // Y is structurally equivalent to X.
  n: number = 0
  s: string = ''
}

let x = new X()
let y = new Y()

console.log('Assign X to Y')
y = x

console.log('Assign Y to X')
x = y

function foo(x: X) {
  console.log(x.n + x.s)
}

// X and Y are equivalent because their public API is equivalent. Therefore, the second call is allowed.
foo(new X())
foo(new Y())

ArkTS

interface I1 {
  f(): string
}

type I2 = I1 // I2 is an alias for I1.

class B {
  n: number = 0
  s: string = ''
}

// D is derived from B, which explicitly set subtype/supertype relations.
class D extends B {
  constructor() {
    super()
  }
}

let b = new B()
let d = new D()

console.log('Assign D to B')
b = d // OK. B is the superclass of D.

// An attempt to assign b to d will result in a compile-time error.
// d = b

interface Z {
   n: number
   s: string
}

// X implements interface Z, which makes relation between X and Y explicit.
class X implements Z {
  n: number = 0
  s: string = ''
}

// Y implements interface Z, which makes relation between X and Y explicit.
class Y implements Z {
  n: number = 0
  s: string = ''
}

let x: Z = new X()
let y: Z = new Y()

console.log('Assign X to Y')
y = x // ok, both are of the same type

console.log('Assign Y to X')
x = y // ok, both are of the same type

function foo(c: Z): void {
  console.log(c.n + c.s)
}

// X and Y implement the same interface. Therefore both calls are allowed.
foo(new X())
foo(new Y())

Recipe: Type Inference in Case of Generic Function Calls Is Limited

Rule: arkts-no-inferred-generic-params

Severity: error

ArkTS allows to omit generic type parameters if it is possible to infer the concrete types from the parameters passed to the function. A compile-time error occurs otherwise. In particular, inference of generic type parameters based only on function return types is prohibited.

TypeScript

function choose<T>(x: T, y: T): T {
  return Math.random() < 0.5 ? x: y
}

let x = choose(10, 20)   // OK. choose<number>(...) is inferred.
let y = choose('10', 20) // Compile-time error

function greet<T>(): T {
  return 'Hello' as T
}
let z = greet() // Type of T is inferred as "unknown".

ArkTS

function choose<T>(x: T, y: T): T {
  return Math.random() < 0.5 ? x: y
}

let x = choose(10, 20)   // OK. choose<number>(...) is inferred.
let y = choose('10', 20) // Compile-time error

function greet<T>(): T {
  return 'Hello' as T
}
let z = greet<string>()

Recipe: RegExp Literals Are Not Supported

Rule: arkts-no-regexp-literals

Severity: error

Currently, ArkTS does not support regular expression literals. Use library calls with string literals instead.

TypeScript

let regex: RegExp = /bc*d/

ArkTS

let regex: RegExp = new RegExp('bc*d')

Recipe: Object Literal Must Correspond to Some Explicitly Declared Class or Interface

Rule: arkts-no-untyped-obj-literals

Severity: error

ArkTS supports usage of object literals if the compiler can infer to the classes or interfaces that such literals correspond to. Otherwise, a compile-time error occurs.

Using literals to initialize classes and interfaces is specifically not supported in the following contexts:

Initialization of anything that has any, Object, or object type
Initialization of classes or interfaces with methods
Initialization of classes which declare a constructor with parameters
Initialization of classes with readonly fields

Example 1

TypeScript

let o1 = {n: 42, s: 'foo'}
let o2: Object = {n: 42, s: 'foo'}
let o3: object = {n: 42, s: 'foo'}

let oo: Object[] = [{n: 1, s: '1'}, {n: 2, s: '2'}]

ArkTS

class C1 {
  n: number = 0
  s: string = ''
}

let o1: C1 = {n: 42, s: 'foo'}
let o2: C1 = {n: 42, s: 'foo'}
let o3: C1 = {n: 42, s: 'foo'}

let oo: C1[] = [{n: 1, s: '1'}, {n: 2, s: '2'}]

Example 2

TypeScript

class C2 {
  s: string
  constructor(s: string) {
    this.s = 's =' + s
  }
}
let o4: C2 = {s: 'foo'}

ArkTS

class C2 {
  s: string
  constructor(s: string) {
    this.s = 's =' + s
  }
}
let o4 = new C2('foo')

Example 3

TypeScript

class C3 {
  readonly n: number = 0
  readonly s: string = ''
}
let o5: C3 = {n: 42, s: 'foo'}

ArkTS

class C3 {
  n: number = 0
  s: string = ''
}
let o5: C3 = {n: 42, s: 'foo'}

Example 4

TypeScript

abstract class A {}
let o6: A = {}

ArkTS

abstract class A {}
class C extends A {}
let o6: C = {} // Or let o6: C = new C()

Example 5

TypeScript

class C4 {
  n: number = 0
  s: string = ''
  f() {
    console.log('Hello')
  }
}
let o7: C4 = {n: 42, s: 'foo', f: () => {}}

ArkTS

class C4 {
  n: number = 0
  s: string = ''
  f() {
    console.log('Hello')
  }
}
let o7 = new C4()
o7.n = 42
o7.s = 'foo'

Example 6

TypeScript

class Point {
  x: number = 0
  y: number = 0
}

function id_x_y(o: Point): Point {
  return o
}

// Structural typing is used to deduce that p is Point.
let p = {x: 5, y: 10}
id_x_y(p)

// A literal can be contextually (i.e., implicitly) typed as Point.
id_x_y({x: 5, y: 10})

ArkTS

class Point {
  x: number = 0
  y: number = 0

    // constructor() is used before literal initialization to create a valid object.
    // Since there is no other Point constructors, constructor() is automatically added by compiler.
}

function id_x_y(o: Point): Point {
  return o
}

// Explicit type is required for literal initialization.
let p: Point = {x: 5, y: 10}
id_x_y(p)

// id_x_y expects Point explicitly. A new instance of Point is initialized with the literal.
id_x_y({x: 5, y: 10})

See also

Recipe: Object Literals Cannot Be Used as Type Declarations
Recipe: Array Literals Must Contain Elements of Only Inferrable Types

Recipe: Object Literals Cannot Be Used as Type Declarations

Rule: arkts-no-obj-literals-as-types

Severity: error

ArkTS does not support the usage of object literals to declare types in place. Declare classes and interfaces explicitly instead.

TypeScript

let o: {x: number, y: number} = {
  x: 2,
  y: 3
}

type S = Set<{x: number, y: number}>

ArkTS

class O {
  x: number = 0
  y: number = 0
}

let o: O = {x: 2, y: 3}

type S = Set<O>

See also

Recipe: Object Literal Must Correspond to Some Explicitly Declared Class or Interface
Recipe: Array Literals Must Contain Elements of Only Inferrable Types

Recipe: Array Literals Must Contain Elements of Only Inferrable Types

Rule: arkts-no-noninferrable-arr-literals

Severity: error

Basically, ArkTS infers the type of an array literal as a union type of its contents. However, a compile-time error occurs if there is at least one element with a non-inferrable type (for example, untyped object literal).

TypeScript

let a = [{n: 1, s: '1'}, {n: 2, s: '2'}]

ArkTS

class C {
  n: number = 0
  s: string = ''
}

let a1 = [{n: 1, s: '1'} as C, {n: 2, s: '2'} as C] // a1 is of type "C[]"
let a2: C[] = [{n: 1, s: '1'}, {n: 2, s: '2'}]    // a2 is of type "C[]"

See also

Recipe: Object Literal Must Correspond to Some Explicitly Declared Class or Interface
Recipe: Object Literals Cannot Be Used as Type Declarations

Recipe: Use Arrow Functions Instead of Function Expressions

Rule: arkts-no-func-expressions

Severity: error

ArkTS does not support function expressions. Use arrow functions instead to specify explicitly.

TypeScript

let f = function (s: string) {
  console.log(s)
}

ArkTS

let f = (s: string) => {
  console.log(s)
}

Recipe: Use Generic Functions Instead of Generic Arrow Functions

Rule: arkts-no-generic-lambdas

Severity: error

ArkTS does not support generic arrow functions. Use normal generic functions instead.

TypeScript

let generic_arrow_func = <T extends String> (x: T) => { return x }

generic_arrow_func('string')

ArkTS

function generic_func<T extends String>(x: T): T {
  return x
}

generic_func<String>('string')

Recipe: Class Literals Are Not Supported

Rule: arkts-no-class-literals

Severity: error

ArkTS does not support class literals. Introduce new named class types explicitly.

TypeScript

const Rectangle = class {
  constructor(height: number, width: number) {
    this.height = height
    this.width = width
  }

  height
  width
}

const rectangle = new Rectangle(0.0, 0.0)

ArkTS

class Rectangle {
  constructor(height: number, width: number) {
    this.height = height
    this.width = width
  }

  height: number
  width: number
}

const rectangle = new Rectangle(0.0, 0.0)

Recipe: Classes Cannot Be Specified in the `implements` Clause

Rule: arkts-implements-only-iface

Severity: error

ArkTS does not allow to specify a class in the implements clause. Only interfaces may be specified.

TypeScript

class C {
  foo() {}
}

class C1 implements C {
  foo() {}
}

ArkTS

interface C {
  foo(): void
}

class C1 implements C {
  foo() {}
}

Recipe: Reassigning Object Methods Is Not Supported

Rule: arkts-no-method-reassignment

Severity: error

ArkTS does not support reassigning a method for objects. In the statically typed languages, the layout of objects is fixed and all instances of the same object must share the same code of each method.

If you need to add specific behavior for certain objects, you can create separate wrapper functions or use inheritance.

TypeScript

class C {
  foo() {
    console.log('foo')
  }
}

function bar() {
  console.log('bar')
}

let c1 = new C()
let c2 = new C()
c2.foo = bar

c1.foo() // foo
c2.foo() // bar

ArkTS

class C {
  foo() {
    console.log('foo')
  }
}

class Derived extends C {
  foo() {
    console.log('Extra')
    super.foo()
  }
}

function bar() {
  console.log('bar')
}

let c1 = new C()
let c2 = new C()
c1.foo() // foo
c2.foo() // foo

let c3 = new Derived()
c3.foo() // Extra foo

Recipe: Only the `as T` Syntax Is Supported for Type Casting

Rule: arkts-as-casts

Severity: error

ArkTS supports the keyword as as the only syntax for type casting. Incorrect casting causes a compile-time error or runtime ClassCastException. The <type> syntax for type casting is not supported.

Use the expression new ... instead of as if a primitive type (such as a number or a boolean) must be cast to the reference type.

TypeScript

class Shape {}
class Circle extends Shape {x: number = 5}
class Square extends Shape {y: string = 'a'}

function createShape(): Shape {
  return new Circle()
}

let c1 = <Circle> createShape()

let c2 = createShape() as Circle

// No report is provided during compilation or runtime if casting is wrong.
let c3 = createShape() as Square
console.log(c3.y) // undefined

// Important corner case for casting primitives to the boxed counterparts:
// The left operand is not properly boxed here in runtime
// because "as" has no runtime effect in TypeScript.
let e1 = (5.0 as Number) instanceof Number // false

// A Number object is created and instanceof works as expected.
let e2 = (new Number(5.0)) instanceof Number // True

ArkTS

class Shape {}
class Circle extends Shape {x: number = 5}
class Square extends Shape {y: string = 'a'}

function createShape(): Shape {
  return new Circle()
}

let c2 = createShape() as Circle

// ClassCastException is thrown during runtime.
let c3 = createShape() as Square

// A Number object is created and instanceof works as expected.
let e2 = (new Number(5.0)) instanceof Number // True

Recipe: JSX Expressions Are Not Supported

Rule: arkts-no-jsx

Severity: error

Do not use JSX since no alternative is provided to rewrite it.

Recipe: Unary Operators `+`, `-`, and `~` Work Only on Numbers

Rule: arkts-no-polymorphic-unops

Severity: error

ArkTS allows unary operators to work on numeric types only. A compile-time error occurs if these operators are applied to a non-numeric type. Unlike in TypeScript, implicit casting of strings in this context is not supported and casting must be done explicitly.

TypeScript

let a = +5        // 5 as number
let b = +'5'      // 5 as number
let c = -5        // -5 as number
let d = -'5'      // -5 as number
let e = ~5        // -6 as number
let f = ~'5'      // -6 as number
let g = +'string' // NaN as number

function returnTen(): string {
  return '-10'
}

function returnString(): string {
  return 'string'
}

let x = +returnTen()    // -10 as number
let y = +returnString() // NaN

ArkTS

let a = +5        // 5 as number
let b = +'5'      // Compile-time error
let c = -5        // -5 as number
let d = -'5'      // Compile-time error
let e = ~5        // -6 as number
let f = ~'5'      // Compile-time error
let g = +'string' // Compile-time error

function returnTen(): string {
  return '-10'
}

function returnString(): string {
  return 'string'
}

let x = +returnTen()    // Compile-time error
let y = +returnString() // Compile-time error

Recipe: `delete` Operator Is Not Supported

Rule: arkts-no-delete

Severity: error

ArkTS assumes that object layout is known at compile time and cannot be changed at runtime. Therefore, the operation of deleting a property makes no sense.

TypeScript

class Point {
  x?: number = 0.0
  y?: number = 0.0
}

let p = new Point()
delete p.y

ArkTS

// To mimic the original semantics, you may declare a nullable type and assign null to mark the value absence.

class Point {
  x: number | null = 0
  y: number | null = 0
}

let p = new Point()
p.y = null

See also

Recipe: Objects with Property Names That Are Not Identifiers Are Not Supported
Recipe: Symbol() Is Not Supported
Recipe: Indexed Access Is Not Supported for Fields
Recipe: typeof Operator Is Allowed Only in Expression Contexts
Recipe: in Operator Is Not Supported

Recipe: `typeof` Operator Is Allowed Only in Expression Contexts

Rule: arkts-no-type-query

Severity: error

ArkTS supports the typeof operator only in the expression context. Using typeof to specify type notations is not supported.

TypeScript

let n1 = 42
let s1 = 'foo'
console.log(typeof n1) // 'number'
console.log(typeof s1) // 'string'
let n2: typeof n1
let s2: typeof s1

ArkTS

let n1 = 42
let s1 = 'foo'
console.log(typeof n1) // 'number'
console.log(typeof s1) // 'string'
let n2: number
let s2: string

See also

Recipe: Objects with Property Names That Are Not Identifiers Are Not Supported
Recipe: Symbol() Is Not Supported
Recipe: Indexed Access Is Not Supported for Fields
Recipe: delete Operator Is Not Supported
Recipe: in Operator Is Not Supported
Recipe: Usage of Standard Libraries Is Restricted

Recipe: `instanceof` Operator Is Partially Supported

Rule: arkts-instanceof-ref-types

Severity: error

In TypeScript, the left-hand side of an instanceof expression must be of the type any, an object type, or a type parameter. Otherwise, the result is false. In ArkTS, the left-hand side of an expression may be of any reference type, for example, an object, an array, or a function. Otherwise, a compile-time error occurs. In addition, the left operand in ArkTS cannot be a type. It must be an object instance.

Recipe: `in` Operator Is Not Supported

Rule: arkts-no-in

Severity: error

ArkTS does not support the operator in. This operator makes little sense since the object layout is known at compile time and cannot be changed at runtime. Use instanceof as a workaround if you want to check whether certain class members exist.

TypeScript

class Person {
  name: string = ''
}
let p = new Person()

let b = 'name' in p // true

ArkTS

class Person {
  name: string = ''
}
let p = new Person()

let b = p instanceof Person // True. "name" is guaranteed to be present.

See also

Recipe: Objects with Property Names That Are Not Identifiers Are Not Supported
Recipe: Symbol() Is Not Supported
Recipe: Indexed Access Is Not Supported for Fields
Recipe: delete Operator Is Not Supported
Recipe: typeof Operator Is Allowed Only in Expression Contexts
Recipe: Usage of Standard Libraries Is Restricted

Recipe: Destructuring Assignment Is Not Supported

Rule: arkts-no-destruct-assignment

Severity: error

ArkTS does not support destructuring assignment. Use other idioms (for example, a temporary variable, where applicable) instead.

TypeScript

let [one, two] = [1, 2]; // Semicolon is required here
[one, two] = [two, one]

let head, tail
[head, ...tail] = [1, 2, 3, 4]

ArkTS

let arr: number[] = [1, 2]
let one = arr[0]
let two = arr[1]

let tmp = one
one = two
two = tmp

let data: Number[] = [1, 2, 3, 4]
let head = data[0]
let tail: Number[] = []
for (let i = 1; i < data.length; ++i) {
  tail.push(data[i])
}

Recipe: Comma Operator `,` Is Supported Only in `for` Loops

Rule: arkts-no-comma-outside-loops

Severity: error

ArkTS supports the comma operator , only in for loops. In other cases, the comma operator is useless as it makes the execution order harder to understand.

Please note that this rule is applied only to the "comma operator". Other cases, when comma is used to delimit variable declarations or parameters of a function call, are of course allowed.

TypeScript

for (let i = 0, j = 0; i < 10; ++i, j += 2) {
  // ...
}

let x = 0
x = (++x, x++) // 1

ArkTS

for (let i = 0, j = 0; i < 10; ++i, j += 2) {
  // ...
}

// Use the explicit execution order instead of the comma operator.
let x = 0
++x
x = x++

Recipe: Destructuring Variable Declarations Are Not Supported

Rule: arkts-no-destruct-decls

Severity: error

ArkTS does not support destructuring variable declarations. This is a dynamic feature relying on structural compatibility. In addition, names in destructuring declarations must be equal to properties within destructed classes.

TypeScript

class Point {
  x: number = 0.0
  y: number = 0.0
}

function returnZeroPoint(): Point {
  return new Point()
}

let {x, y} = returnZeroPoint()

ArkTS

class Point {
  x: number = 0.0
  y: number = 0.0
}

function returnZeroPoint(): Point {
  return new Point()
}

// Create an intermediate object and work with it field by field without name restrictions.
let zp = returnZeroPoint()
let x = zp.x
let y = zp.y

Recipe: Type Annotation in the Catch Clause Is Not Supported

Rule: arkts-no-types-in-catch

Severity: error

In TypeScript, the catch clause variable type annotation must be any or unknown if specified. As ArkTS does not support these types, omit type annotations.

TypeScript

try {
  // ...
} catch (a: unknown) {
  // Handle errors.
}

ArkTS

try {
  // ...
} catch (a) {
  // Handle errors.
}

See also

Recipe: throw Statements Do Not Accept Values of Arbitrary Types

Recipe: `for .. in` Is Not Supported

Rule: arkts-no-for-in

Severity: error

ArkTS does not support iteration over object contents by the for .. in loop. For objects, iteration over properties at runtime is considered redundant because object layout is known at compile time and cannot change at runtime. For arrays, use the regular for loop for iteration.

TypeScript

let a: string[] = ['1.0', '2.0', '3.0']
for (let i in a) {
  console.log(a[i])
}

ArkTS

let a: string[] = ['1.0', '2.0', '3.0']
for (let i = 0; i < a.length; ++i) {
  console.log(a[i])
}

Recipe: Mapped Type Expression Is Not Supported

Rule: arkts-no-mapped-types

Severity: error

ArkTS does not support mapped types. Use other language idioms and regular classes to achieve the same behaviour.

TypeScript

type OptionsFlags<Type> = {
  [Property in keyof Type]: boolean
}

ArkTS

class C {
  n: number = 0
  s: string = ''
}

class CFlags {
  n: boolean = false
  s: boolean = false
}

Recipe: `with` Statement Is Not Supported

Rule: arkts-no-with

Severity: error

ArkTS does not support the with statement. Use other language idioms to achieve the same behaviour.

TypeScript

with (Math) { // Compile-time error, but JavaScript code can still be emitted.
  let r: number = 42;
  let area: number = PI * r * r;
}

ArkTS

let r: number = 42;
let area: number = Math.PI * r * r;

Recipe: `throw` Statements Do Not Accept Values of Arbitrary Types

Rule: arkts-limited-throw

Severity: error

ArkTS supports throwing only objects of the class Error or any derived class. Throwing an arbitrary type (for example, a number or string) is prohibited.

TypeScript

throw 4
throw ''
throw new Error()

ArkTS

throw new Error()

Recipe: Function Return Type Inference Is Limited

Rule: arkts-no-implicit-return-types

Severity: error

ArkTS supports type inference for function return types, but this functionality is currently restricted. In particular, when the expression in the return statement is a call to a function or method whose return value type is omitted, a compile-time error occurs. If this is the case, specify the return type explicitly.

TypeScript

// Compile-time error when noImplicitAny is enabled.
function f(x: number) {
  if (x <= 0) {
    return x;
  }
  return g(x);
}

// Compile-time error when noImplicitAny is enabled.
function g(x: number) {
  return f(x - 1);
}

function doOperation(x: number, y: number) {
  return x + y;
}

f(10);
doOperation(2, 3);

ArkTS

// An explicit return type is required.
function f(x: number): number {
  if (x <= 0) {
    return x;
  }
  return g(x);
}

// Return type may be omitted. It is inferred from f's explicit type.
function g(x: number): number {
  return f(x - 1);
}

// In this case, the return type will be inferred.
function doOperation(x: number, y: number) {
  return x + y;
}

f(10);
doOperation(2, 3);

Recipe: Destructuring Parameter Declarations Are Not Supported

Rule: arkts-no-destruct-params

Severity: error

ArkTS requires parameters to be passed directly to the function, and local names to be assigned manually.

TypeScript

function drawText({ text = '', location: [x, y] = [0, 0], bold = false }) {
  text;
  x;
  y;
  bold;
}

drawText({ text: 'Hello, world!', location: [100, 50], bold: true })

ArkTS

function drawText(text: String, location: number[], bold: boolean) {
  let x = location[0];
  let y = location[1];
  text;
  x;
  y;
  bold;
}

function main() {
  drawText('Hello, world!', [100, 50], true);
}

Recipe: Nested Functions Are Not Supported

Rule: arkts-no-nested-funcs

Severity: error

ArkTS does not support nested functions. Use lambdas instead.

TypeScript

function addNum(a: number, b: number): void {

    // Nested function
  function logToConsole(message: string): void {
    console.log(message)
  }

  let result = a + b

    // Invoke the nested function.
  logToConsole('result is ' + result)
}

ArkTS

function addNum(a: number, b: number): void {
    // Use lambda instead of a nested function.
  let logToConsole: (message: string) => void = (message: string): void => {
    console.log(message)
  }

  let result = a + b

  logToConsole('result is ' + result)
}

Recipe: Using `this` Inside Stand-Alone Functions Is Not Supported

Rule: arkts-no-standalone-this

Severity: error

ArkTS does not support the usage of this inside stand-alone functions and inside static methods. this can be used in instance methods only.

TypeScript

function foo(i: string) {
    this.count = i // Compile-time error only when noImplicitThis is enabled.
}

class A {
  count: string = 'a'
  m = foo
}

let a = new A();
console.log(a.count); // Prints "a".
a.m(b);
console.log(a.count); // Prints "b".

ArkTS

class A {
  count: string = 'a'
  m(i: string): void {
    this.count = i;
  }
}

function main(): void {
  let a = new A();
  console.log(a.count);  // Prints "a".
  a.m('b');
  console.log(a.count);  // Prints "b".
}

See also

Recipe: Function.apply, Function.bind, and Function.call Are Not Supported

Recipe: Generator Functions Are Not Supported

Rule: arkts-no-generators

Severity: error

Currently, ArkTS does not support generator functions. Use the async/await mechanism for multitasking.

TypeScript

function* counter(start: number, end: number) {
  for (let i = start; i <= end; i++) {
    yield i
  }
}

for (let num of counter(1, 5)) {
  console.log(num)
}

ArkTS

async function complexNumberProcessing(str: string): Promise<string> {
  // ...
  return str
}

async function foo() {
  for (let i = 1; i <= 5; i++) {
    console.log(await complexNumberProcessing(i))
  }
}

foo()

Recipe: Type Guarding Is Supported with `instanceof` and `as`

Rule: arkts-no-is

Severity: error

ArkTS does not support the is operator, which must be replaced by the instanceof operator. Note that the fields of an object must be cast to the appropriate type with the as operator before being used.

TypeScript

class Foo {
  foo: string = ''
  common: string = ''
}

class Bar {
  bar: string = ''
  common: string = ''
}

function isFoo(arg: any): arg is Foo {
  return arg.foo !== undefined
}

function doStuff(arg: Foo | Bar) {
  if (isFoo(arg)) {
    console.log(arg.foo)  // OK
    console.log(arg.bar)  // Compile-time error
  } else {
    console.log(arg.foo)  // Compile-time error
    console.log(arg.bar)  // OK
  }
}

doStuff({ foo: 123, common: '123' })
doStuff({ bar: 123, common: '123' })

ArkTS

class Foo {
  foo: string = ''
  common: string = ''
}

class Bar {
  bar: string = ''
  common: string = ''
}

function isFoo(arg: Object): boolean {
  return arg instanceof Foo
}

function doStuff(arg: Object): void {
  if (isFoo(arg)) {
    let fooArg = arg as Foo
    console.log(fooArg.foo)   // OK
    console.log(arg.bar)    // Compile-time error
  } else {
    let barArg = arg as Bar
    console.log(arg.foo)    // Compile-time error
    console.log(barArg.bar)   // OK
  }
}

function main(): void {
  doStuff(new Foo())
  doStuff(new Bar())
}

Recipe: It is possible to spread only arrays or classes derived from arrays into the rest parameter or array literals

Rule: arkts-no-spread

Severity: error

The only supported scenario for the spread operator is to spread an array or class derived from array into the rest parameter or array literal. Otherwise, manually "unpack" data from arrays and objects, where necessary. All typed arrays from the standard library (for example, Int32Array) are also supported.

TypeScript

function foo(x: number, y: number, z: number) {
  // ...
}

let args: [number, number, number] = [0, 1, 2];
foo(...args);

ArkTS

function log_numbers(x: number, y: number, z: number) {
  // ...
}

let numbers: number[] = [1, 2, 3];
log_numbers(numbers[0], numbers[1], numbers[2]);

TypeScript

let point2d = { x: 1, y: 2 };
let point3d = { ...point2d, z: 3 };

ArkTS

class Point2D {
  x: number = 0; y: number = 0
}

class Point3D {
  x: number = 0; y: number = 0; z: number = 0
  constructor(p2d: Point2D, z: number) {
    this.x = p2d.x;
    this.y = p2d.y;
    this.z = z;
  }
}

let p3d = new Point3D({ x: 1, y: 2 } as Point2D, 3);

class DerivedFromArray extends Uint16Array {};

let arr1 = [1, 2, 3];
let arr2 = new Uint16Array([4, 5, 6]);
let arr3 = new DerivedFromArray([7, 8, 9]);
let arr4 = [...arr1, 10, ...arr2, 11, ...arr3];

Recipe: Interface Cannot Extend Interfaces with the Same Method

Rule: arkts-no-extend-same-prop

Severity: error

In TypeScript, an interface that extends two other interfaces with the same method must declare that method with a combined return type. It is not allowed in ArkTS because ArkTS does not allow an interface to contain two methods with signatures that are not distinguishable, for example, two methods that have the same parameter lists but different return types.

TypeScript

interface Mover {
  getStatus(): { speed: number }
}
interface Shaker {
  getStatus(): { frequency: number }
}

interface MoverShaker extends Mover, Shaker {
  getStatus(): {
    speed: number
    frequency: number
  }
}

class C implements MoverShaker {
  private speed: number = 0
  private frequency: number = 0

  getStatus() {
    return { speed: this.speed, frequency: this.frequency }
  }
}

ArkTS

class MoveStatus {
  public speed: number
  constructor() {
    this.speed = 0
  }
}
interface Mover {
  getMoveStatus(): MoveStatus
}

class ShakeStatus {
  public frequency: number
  constructor() {
    this.frequency = 0
  }
}
interface Shaker {
  getShakeStatus(): ShakeStatus
}

class MoveAndShakeStatus {
  public speed: number
  public frequency: number
  constructor() {
    this.speed = 0
    this.frequency = 0
  }
}

class C implements Mover, Shaker {
  private move_status: MoveStatus
  private shake_status: ShakeStatus

  constructor() {
    this.move_status = new MoveStatus()
    this.shake_status = new ShakeStatus()
  }

  public getMoveStatus(): MoveStatus {
    return this.move_status
  }

  public getShakeStatus(): ShakeStatus {
    return this.shake_status
  }

  public getStatus(): MoveAndShakeStatus {
    return {
      speed: this.move_status.speed,
      frequency: this.shake_status.frequency
    }
  }
}

Recipe: Declaration Merging Is Not Supported

Rule: arkts-no-decl-merging

Severity: error

ArkTS does not support merging declarations. Keep all definitions of classes and interfaces compact in the codebase.

TypeScript

interface Document {
  createElement(tagName: any): Element
}

interface Document {
  createElement(tagName: string): HTMLElement
}

interface Document {
  createElement(tagName: number): HTMLDivElement
  createElement(tagName: boolean): HTMLSpanElement
  createElement(tagName: string, value: number): HTMLCanvasElement
}

ArkTS

interface Document {
  createElement(tagName: number): HTMLDivElement
  createElement(tagName: boolean): HTMLSpanElement
  createElement(tagName: string, value: number): HTMLCanvasElement
  createElement(tagName: string): HTMLElement
  createElement(tagName: Object): Element
}

Recipe: Interfaces Cannot Extend Classes

Rule: arkts-extends-only-class

Severity: error

ArkTS does not support interfaces that extend classes. Interfaces can extend only interfaces.

TypeScript

class Control {
  state: number = 0
}

interface SelectableControl extends Control {
  select(): void
}

ArkTS

interface Control {
  state: number
}

interface SelectableControl extends Control {
  select(): void
}

Recipe: Constructor Function Type Is Not Supported

Rule: arkts-no-ctor-signatures-funcs

Severity: error

ArkTS does not support the usage of the constructor function type. Use lambdas instead.

TypeScript

class Person {
  constructor(
    name: string,
    age: number
  ) {}
}
type PersonCtor = new (name: string, age: number) => Person

function createPerson(Ctor: PersonCtor, name: string, age: number): Person
{
  return new Ctor(name, age)
}

const person = createPerson(Person, 'John', 30)

ArkTS

class Person {
  constructor(
    name: string,
    age: number
  ) {}
}
type PersonCtor = (n: string, a: number) => Person

function createPerson(Ctor: PersonCtor, n: string, a: number): Person {
  return Ctor(n, a)
}

let Impersonizer: PersonCtor = (n: string, a: number): Person => {
  return new Person(n, a)
}

const person = createPerson(Impersonizer, 'John', 30)

Recipe: Enumeration Members Can Be Initialized Only with Compile Time Expressions of the Same Type

Rule: arkts-no-enum-mixed-types

Severity: error

ArkTS does not support initializing members of enumerations with expressions that are evaluated during program runtime. Besides, all explicitly set initializers must be of the same type.

TypeScript

enum E1 {
  A = 0xa,
  B = 0xb,
  C = Math.random(),
  D = 0xd,
  E // 0xe inferred
}

enum E2 {
  A = 0xa,
  B = '0xb',
  C = 0xc,
  D = '0xd'
}

ArkTS

enum E1 {
  A = 0xa,
  B = 0xb,
  C = 0xc,
  D = 0xd,
  E // 0xe inferred
}

enum E2 {
  A = '0xa',
  B = '0xb',
  C = '0xc',
  D = '0xd'
}

Recipe: `enum` Declaration Merging Is Not Supported

Rule: arkts-no-enum-merging

Severity: error

ArkTS does not support merging declarations for enum. Keep the declaration of each enum compact in the codebase.

TypeScript

enum ColorSet {
  RED,
  GREEN
}
enum ColorSet {
  YELLOW = 2
}
enum ColorSet {
  BLACK = 3,
  BLUE
}

ArkTS

enum ColorSet {
  RED,
  GREEN,
  YELLOW,
  BLACK,
  BLUE
}

Recipe: Namespaces Cannot Be Used as Objects

Rule: arkts-no-ns-as-obj

Severity: error

ArkTS does not support the usage of namespaces as objects. Classes or modules can be interpreted as analogs of namespaces.

TypeScript

namespace MyNamespace {
  export let x: number
}

let m = MyNamespace
m.x = 2

ArkTS

namespace MyNamespace {
  export let x: number
}

MyNamespace.x = 2

Recipe: Non-declaration Statements in Namespaces Are Not Supported

Rule: arkts-no-ns-statements

Severity: error

ArkTS does not support statements in namespaces. Use a function to execute statements.

TypeScript

namespace A {
  export let x: number
  x = 1
}

ArkTS

namespace A {
  export let x: number

  export function init() {
    x = 1
  }
}

// Initialization function should be called to execute statements.
A.init()

Recipe: Importing a Module for Side-Effects Only Is Not Supported

Rule: arkts-no-side-effects-imports

Severity: error

ArkTS does not support global variables like window to avoid side-effects during module importing. All variables marked as export can be accessed through the * syntax.

TypeScript

// === module at "path/to/module.ts"
export const EXAMPLE_VALUE = 42

// Set a global variable.
window.MY_GLOBAL_VAR = 'Hello, world!'

// ==== using this module:
import 'path/to/module'

ArkTS

import * as ns from 'path/to/module'

Recipe: `import default as ...` Is Not Supported

Rule: arkts-no-import-default-as

Severity: error

ArkTS does not support the import default as ... syntax. Use explicit import ... from ... instead.

TypeScript

import { default as d } from 'mod'

ArkTS

import d from 'mod'

Recipe: `require` and `import` Assignment Are Not Supported

Rule: arkts-no-require

Severity: error

ArkTS does not support importing via require. It does not support import assignments either. Use the regular import syntax instead.

TypeScript

import m = require('mod')

ArkTS

import * as m from 'mod'

See also

Recipe: export = ... Is Not Supported

Recipe: `export = ...` Is Not Supported

Rule: arkts-no-export-assignment

Severity: error

ArkTS does not support the export = ... syntax. Use the ordinary export and import syntaxes instead.

TypeScript

// module1
export = Point

class Point {
  constructor(x: number, y: number) {}
  static origin = new Point(0, 0)
}

// module2
import Pt = require('module1')

let p = Pt.Point.origin

ArkTS

// module1
export class Point {
  constructor(x: number, y: number) {}
  static origin = new Point(0, 0)
}

// module2
import * as Pt from 'module1'

let p = Pt.Point.origin

See also

Recipe: require and import Assignment Are Not Supported

Recipe: Ambient Module Declaration Is Not Supported

Rule: arkts-no-ambient-decls

Severity: error

ArkTS does not support ambient module declaration because it has its own mechanisms for interoperating with JavaScript.

TypeScript

declare module 'someModule' {
  export function normalize(s: string): string;
}

ArkTS

// Import what you need from the original module.
import { normalize } from "someModule"

See also

Recipe: Wildcards in Module Names Are Not Supported

Rule: arkts-no-module-wildcards

Severity: error

ArkTS does not support wildcards in module names, because import is a compile-time feature in ArkTS, not a runtime feature. Use the ordinary export syntax instead.

TypeScript

// Declaration:
declare module '*!text' {
  const content: string
  export default content
}

// Consuming code:
import fileContent from 'some.txt!text'

ArkTS

// Declaration:
declare namespace N {
  function foo(x: number): number
}

// Consuming code:
import * as m from 'module'
console.log('N.foo called: ' + N.foo(42))

See also

Recipe: Ambient Module Declaration Is Not Supported
Recipe: UMD Is Not Supported

Recipe: UMD Is Not Supported

Rule: arkts-no-umd

Severity: error

ArkTS does not support universal module definitions (UMD), because it does not have the concept of "script" (as opposed to "module"). In addition, import is a compile-time feature in ArkTS, not a runtime feature. Use the ordinary export and import syntaxes instead.

TypeScript

// math-lib.d.ts
export const isPrime(x: number): boolean
export as namespace mathLib

// In script
mathLib.isPrime(2)

ArkTS

// math-lib.d.ts
namespace mathLib {
  export isPrime(x: number): boolean
}

// In program
import { mathLib } from "math-lib"
mathLib.isPrime(2)

See also

Recipe: Wildcards in Module Names Are Not Supported

Recipe: `new.target` Is Not Supported

Rule: arkts-no-new-target

Severity: error

ArkTS does not support new.target, because there is no concept of runtime prototype inheritance in the language. This feature is considered not applicable to static typing.

See also

Recipe: Prototype Assignment Is Not Allowed

Recipe: Definite Assignment Assertions Are Not Supported

Rule: arkts-no-definite-assignment

Severity: warning

ArkTS does not support definite assignment assertions let v!: T because they are considered an excessive compiler hint. Use declarations with initialization instead.

TypeScript

let x!: number // Hint: x will be initialized before usage.

initialize()

function initialize() {
  x = 10
}

console.log('x = ' + x)

ArkTS

function initialize(): number {
  return 10
}

let x: number = initialize()

console.log('x = ' + x)

Recipe: Prototype Assignment Is Not Supported

Rule: arkts-no-prototype-assignment

Severity: error

ArkTS does not support prototype assignment because there is no concept of runtime prototype inheritance in the language. This feature is considered not applicable to static typing. Mechanism of classes and/or interfaces must be used instead to statically "combine" methods to data together.

TypeScript

let C = function(p) {
  this.p = p // Compile-time error only when noImplicitThis is enabled
}

C.prototype = {
  m() {
    console.log(this.p)
  }
}

C.prototype.q = function(r: string) {
  return this.p == r
}

ArkTS

class C {
  p: string = ''
  m() {
    console.log(this.p)
  }
  q(r: string) {
    return this.p == r
  }
}

See also

Recipe: new.target Is Not Supported

Recipe: `globalThis` Is Not Supported

Rule: arkts-no-globalthis

Severity: error

ArkTS does not support both global scope and globalThis because untyped objects with dynamically changed layout are not supported.

TypeScript

// In a global file:
var abc = 100

// Refers to 'abc' from above.
let x = globalThis.abc

ArkTS

// File 1
export let abc: number = 100

// File 2
import * as M from 'file1'

let x = M.abc

See also

Recipe: Declaring Properties on Functions Is Not Supported
Recipe: Usage of Standard Libraries Is Restricted

Recipe: Some Utility Types Are Not Supported

Rule: arkts-no-utility-types

Severity: error

Currently ArkTS does not support utility types from TypeScript extensions to the standard library. Exceptions are Partial, Required, Readonly and Record.

For the type Record<K, V>, the type of an indexing expression rec[index] is of the type V | undefined.

Recipe: Declaring Properties on Functions Is Not Supported

Rule: arkts-no-func-props

Severity: error

ArkTS does not support declaring properties on functions because there is no support for objects with dynamically changing layout. Function objects follow this rule and their layout cannot be changed at runtime.

See also

Recipe: globalThis Is Not Supported

Recipe: `Function.apply`, `Function.bind`, and `Function.call` Are Not Supported

Rule: arkts-no-func-apply-bind-call

Severity: error

ArkTS does not support Function.apply, Function.bind, or Function.call. These APIs are needed in the standard library to explicitly set the parameter this for the called function. In ArkTS, the semantics of this is restricted to the conventional OOP style, and the usage of this in stand-alone functions is prohibited.

See also

Recipe: Using this Inside Stand-Alone Functions Is Not Supported

Recipe: `as const` Assertions Are Not Supported

Rule: arkts-no-as-const

Severity: error

ArkTS does not support as const assertions, because in standard TypeScript as const is used to annotate literals with corresponding literal types, and ArkTS does not support literal types.

TypeScript

// Type 'hello':
let x = 'hello' as const

// Type 'readonly [10, 20]':
let y = [10, 20] as const

// Type '{ readonly text: "hello" }':
let z = { text: 'hello' } as const

ArkTS

// Type 'string':
let x: string = 'hello'

// Type 'number[]':
let y: number[] = [10, 20]

class Label {
  text: string = ''
}

// Type 'Label':
let z: Label = {
  text: 'hello'
}

Recipe: Import Assertions Are Not Supported

Rule: arkts-no-import-assertions

Severity: error

ArkTS does not support import assertions, because import is a compile-time feature in ArkTS, not a runtime feature. So asserting the correctness of imported APIs in runtime does not make sense for the statically typed language. Use the ordinary import syntax instead.

TypeScript

import { obj } from 'something.json' assert { type: 'json' }

ArkTS

// The correctness of importing T will be checked at compile time.
import { something } from 'module'

See also

Recipe: Ambient Module Declaration Is Not Supported
Recipe: UMD Is Not Supported

Recipe: Usage of Standard Libraries Is Restricted

Rule: arkts-limited-stdlib

Severity: error

ArkTS does not support certain APIs in the TypeScript and JavaScript standard libraries. Most of these restricted APIs are used to manipulate objects in a dynamic manner, which is not compatible with static typing. The usage of the following APIs is prohibited:

Properties and functions of the global object: eval

Object: __proto__, __defineGetter__, __defineSetter__, __lookupGetter__, __lookupSetter__, assign, create, defineProperties, defineProperty, freeze, fromEntries, getOwnPropertyDescriptor, getOwnPropertyDescriptors, getOwnPropertySymbols, getPrototypeOf, hasOwnProperty, is, isExtensible, isFrozen, isPrototypeOf, isSealed, preventExtensions, propertyIsEnumerable, seal, setPrototypeOf

Reflect: apply, construct, defineProperty, deleteProperty, getOwnPropertyDescriptor, getPrototypeOf, isExtensible, preventExtensions, setPrototypeOf

Proxy: handler.apply(), handler.construct(), handler.defineProperty(), handler.deleteProperty(), handler.get(), handler.getOwnPropertyDescriptor(), handler.getPrototypeOf(), handler.has(), handler.isExtensible(), handler.ownKeys(), handler.preventExtensions(), handler.set(), handler.setPrototypeOf()

See also

Recipe: Objects with Property Names That Are Not Identifiers Are Not Supported
Recipe: Symbol() Is Not Supported
Recipe: Indexed Access Is Not Supported for Fields
Recipe: typeof Operator Is Allowed Only in Expression Contexts
Recipe: in Operator Is Not Supported
Recipe: globalThis Is Not Supported

Recipe: Strict Type Checking Is Enforced

Rule: arkts-strict-typing

Severity: error

Type checking in ArkTS is not optional. Any code must be explicitly and correctly typed to be compiled and run. When porting code from standard TypeScript, enable the following flags: noImplicitReturns, strictFunctionTypes, strictNullChecks, and strictPropertyInitialization.

TypeScript

// Compile-time error only when noImplicitReturns is enabled.
function foo(s: string): string {
  if (s != '') {
    console.log(s)
    return s
  } else {
    console.log(s)
  }
}

let n: number = null // Compile-time error only when strictNullChecks is enabled.

ArkTS

function foo(s: string): string {
  console.log(s)
  return s
}

let n1: number | null = null
let n2: number = 0

If you cannot initialize an instance property by declaration or in a constructor when defining a class, you can use the definite assignment assertion operator (!) to clear the strictPropertyInitialization error.

However, the use of the definite assignment assertion operator (!) increases the risk of code errors. Therefore, you must ensure that the instance property has been assigned a value before being used. Otherwise, the runtime exceptions may occur.

In addition, the use of the definite assignment assertion operator (!) requires a runtime type check, resulting in additional runtime overhead. It also generates the error warning: arkts-no-definite-assignment at the compile time. Therefore, use it only when quite necessary.

TypeScript

class C {
  name: string  // ֻCompile-time error only when strictPropertyInitialization is enabled.
  age: number   // ֻCompile-time error only when strictPropertyInitialization is enabled.
}

let c = new C()

ArkTS

class C {
  name: string = ''
  age!: number      // warning: arkts-no-definite-assignment

  initAge(age: number) {
    this.age = age
  }
}

let c = new C()
c.initAge(10)

See also

Recipe: Use Explicit Types Instead of any or unknown
Recipe: Disabling Type Checking with In-Place Comments Is Not Allowed

Recipe: Disabling Type Checking with In-Place Comments Is Not Allowed

Rule: arkts-strict-typing-required

Severity: error

Type checking in ArkTS is not optional. Any code must be explicitly and correctly typed to be compiled and run. Disabling type checking in-place with special comments is not allowed. In particular, @ts-ignore and @ts-nocheck annotations are not supported.

TypeScript

// @ts-nocheck
// ...
// Some code with type checking disabled.
// ...

let s1: string = null // No error, since type checking is disabled.

// @ts-ignore
let s2: string = null // No error, since type checking is disabled.

ArkTS

let s1: string | null = null // No error. The types are proper.
let s2: string = null // Compile-time error.

See also

Recipe: Use Explicit Types Instead of any or unknown
Recipe: Strict Type Checking Is Enforced

Recipe: No Dependencies on TypeScript Code Are Allowed

Rule: arkts-no-ts-deps

Severity: error

Currently, the codebase implemented in the standard TypeScript language must not depend on ArkTS by importing an ArkTS codebase. Imports in the reverse direction are supported.

TypeScript

// app.ets
export class C {
  // ...
}

// lib.ts
import { C } from 'app'

ArkTS

// lib1.ets
export class C {
  // ...
}

// lib2.ets
import { C } from 'lib1'

Recipe: Only ArkUI Decorators Are Allowed

Rule: arkts-no-decorators-except-arkui

Severity: warning

Currently, only ArkUI decorators are allowed in ArkTS. Any other decorator will cause a compile-time error.

Recipe: Classes Cannot Be Used as Objects

Rule: arkts-no-classes-as-obj

Severity: error

ArkTS does not support using classes as objects (assigning them to variables, etc.). This is because in ArkTS, a class declaration introduces a new type, not a value.

Recipe: `import` Statements After Other Statements Are Not Allowed

Rule: arkts-no-misplaced-imports

Severity: error

In ArkTS, all import statements should go before all other statements in the program.

TypeScript

class C {
  s: string = ''
  n: number = 0
}

import foo from 'module1'

ArkTS

import foo from 'module1'

class C {
  s: string = ''
  n: number = 0
}

Recipe: Usage of `ESObject` Type Is Restricted

Rule: arkts-limited-esobj

Severity: warning

ArkTS does not allow using ESObject type in some cases. The most part of limitations are put in place in order to prevent spread of dynamic objects in the static codebase. The only scenario where it is permited to use ESObject as type specifier is in local variable declaration. Initialization of variables with ESObject type is also limited. Such variables can only be initialized with values that originate from interop: other ESObject typed variables, any, unknown, variables with anonymous type, etc. It is prohibited to initialize ESObject typed variable with statically typed value. Varaible of type ESObject can only be passed to interop calls and assigned to other variables of type ESObject.

ArkTS

// lib.d.ts
declare function foo(): any;
declare function bar(a: any): number;

// main.ets
let e0: ESObject = foo(); // Compile-time error: 'ESObject' typed variable can only be local

function f() {
  let e1 = foo();        // Compile-time error: type of e1 is 'any'
  let e2: ESObject = 1;  // Compile-time error: can't initialize 'ESObject' with not dynamic values
  let e3: ESObject = {}; // Compile-time error: can't initialize 'ESObject' with not dynamic values
  let e4: ESObject = []; // Compile-time error: can't initialize 'ESObject' with not dynamic values
  let e5: ESObject = ""; // Compile-time error: can't initialize 'ESObject' with not dynamic values
  e5['prop']             // Compile-time error: can't access dynamic properties of 'ESObject'
  e5[1]                  // Compile-time error: can't access dynamic properties of 'ESObject'
  e5.prop                // Compile-time error: can't access dynamic properties of 'ESObject'

  let e6: ESObject = foo(); // OK - explicitly annotated as 'ESObject'
  let e7 = e6;              // OK - initialize 'ESObject' with 'ESObject'
  bar(e7)                   // OK - 'ESObject' is passed to interop call
}

See also

Recipe: Objects with Property Names That Are Not Identifiers Are Not Supported
Recipe: Symbol() Is Not Supported
Recipe: Indexed Access Is Not Supported for Fields
Recipe: typeof Operator Is Allowed Only in Expression Contexts
Recipe: in Operator Is Not Supported
Recipe: globalThis Is Not Supported

TypeScript to ArkTS Cookbook

Recipes Summarized

Static Typing Is Enforced

Changing Object Layout in Runtime Is Prohibited

Semantics of Operators Is Restricted

Structural Typing Is Not Supported (Yet)

Recipes

Recipe: Objects with Property Names That Are Not Identifiers Are Not Supported

Recipe: Symbol() Is Not Supported

Recipe: Private # Identifiers Are Not Supported

Recipe: Use Unique Names for Types and Namespaces

Recipe: Use let Instead of var

Recipe: Use Explicit Types Instead of any or unknown

Recipe: Use class Instead of a Type with a Call Signature

Recipe: Use class Instead of a Type with a Constructor Signature

Recipe: Only One Static Block Is Supported

Recipe: Indexed Signatures Are Not Supported

Recipe: Use Inheritance Instead of Intersection Types

Recipe: Type Notation Using this Is Not Supported

Recipe: Conditional Types Are Not Supported

Recipe: Declaring Fields in constructor Is Not Supported

Recipe: Constructor Signatures Are Not Supported in Interfaces

Recipe: Indexed Access Types Are Not Supported

Recipe: Indexed Access Is Not Supported for Fields

Recipe: Structural Typing Is Not Supported

Recipe: Type Inference in Case of Generic Function Calls Is Limited

Recipe: RegExp Literals Are Not Supported

Recipe: Object Literal Must Correspond to Some Explicitly Declared Class or Interface

Recipe: Object Literals Cannot Be Used as Type Declarations

Recipe: Array Literals Must Contain Elements of Only Inferrable Types

Recipe: Use Arrow Functions Instead of Function Expressions

Recipe: Use Generic Functions Instead of Generic Arrow Functions

Recipe: Class Literals Are Not Supported

Recipe: Classes Cannot Be Specified in the implements Clause

Recipe: Reassigning Object Methods Is Not Supported

Recipe: Only the as T Syntax Is Supported for Type Casting

Recipe: JSX Expressions Are Not Supported

Recipe: Unary Operators +, -, and ~ Work Only on Numbers

Recipe: delete Operator Is Not Supported

Recipe: typeof Operator Is Allowed Only in Expression Contexts

Recipe: instanceof Operator Is Partially Supported

Recipe: in Operator Is Not Supported

Recipe: Destructuring Assignment Is Not Supported

Recipe: Comma Operator , Is Supported Only in for Loops

Recipe: Destructuring Variable Declarations Are Not Supported

Recipe: Type Annotation in the Catch Clause Is Not Supported

Recipe: for .. in Is Not Supported

Recipe: Mapped Type Expression Is Not Supported

Recipe: with Statement Is Not Supported

Recipe: throw Statements Do Not Accept Values of Arbitrary Types

Recipe: Function Return Type Inference Is Limited

Recipe: Destructuring Parameter Declarations Are Not Supported

Recipe: Nested Functions Are Not Supported

Recipe: Using this Inside Stand-Alone Functions Is Not Supported

Recipe: Generator Functions Are Not Supported

Recipe: Type Guarding Is Supported with instanceof and as

Recipe: It is possible to spread only arrays or classes derived from arrays into the rest parameter or array literals

Recipe: Interface Cannot Extend Interfaces with the Same Method

Recipe: Declaration Merging Is Not Supported

Recipe: Interfaces Cannot Extend Classes

Recipe: Constructor Function Type Is Not Supported

Recipe: Enumeration Members Can Be Initialized Only with Compile Time Expressions of the Same Type

Recipe: enum Declaration Merging Is Not Supported

Recipe: Namespaces Cannot Be Used as Objects

Recipe: Non-declaration Statements in Namespaces Are Not Supported

Recipe: Importing a Module for Side-Effects Only Is Not Supported

Recipe: import default as ... Is Not Supported

Recipe: require and import Assignment Are Not Supported

Recipe: export = ... Is Not Supported

Recipe: Ambient Module Declaration Is Not Supported

Recipe: Wildcards in Module Names Are Not Supported

Recipe: UMD Is Not Supported

Recipe: new.target Is Not Supported

Recipe: Definite Assignment Assertions Are Not Supported

Recipe: Prototype Assignment Is Not Supported

Recipe: globalThis Is Not Supported

Recipe: Some Utility Types Are Not Supported

Recipe: Declaring Properties on Functions Is Not Supported

Recipe: Function.apply, Function.bind, and Function.call Are Not Supported

Recipe: as const Assertions Are Not Supported

Recipe: `Symbol()` Is Not Supported

Recipe: Private `#` Identifiers Are Not Supported

Recipe: Use `let` Instead of `var`

Recipe: Use Explicit Types Instead of `any` or `unknown`

Recipe: Use `class` Instead of a Type with a Call Signature

Recipe: Use `class` Instead of a Type with a Constructor Signature

Recipe: Type Notation Using `this` Is Not Supported

Recipe: Declaring Fields in `constructor` Is Not Supported

Recipe: Classes Cannot Be Specified in the `implements` Clause

Recipe: Only the `as T` Syntax Is Supported for Type Casting

Recipe: Unary Operators `+`, `-`, and `~` Work Only on Numbers

Recipe: `delete` Operator Is Not Supported

Recipe: `typeof` Operator Is Allowed Only in Expression Contexts

Recipe: `instanceof` Operator Is Partially Supported

Recipe: `in` Operator Is Not Supported

Recipe: Comma Operator `,` Is Supported Only in `for` Loops

Recipe: `for .. in` Is Not Supported

Recipe: `with` Statement Is Not Supported

Recipe: `throw` Statements Do Not Accept Values of Arbitrary Types

Recipe: Using `this` Inside Stand-Alone Functions Is Not Supported

Recipe: Type Guarding Is Supported with `instanceof` and `as`

Recipe: `enum` Declaration Merging Is Not Supported

Recipe: `import default as ...` Is Not Supported

Recipe: `require` and `import` Assignment Are Not Supported

Recipe: `export = ...` Is Not Supported

Recipe: `new.target` Is Not Supported

Recipe: `globalThis` Is Not Supported

Recipe: `Function.apply`, `Function.bind`, and `Function.call` Are Not Supported

Recipe: `as const` Assertions Are Not Supported

Recipe: `import` Statements After Other Statements Are Not Allowed

Recipe: Usage of `ESObject` Type Is Restricted