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TypeScript provides several utility types to facilitate common type transformations. These utilities are available globally. Partial Constructs…","html":"

Introduction

\n

TypeScript provides several utility types to facilitate common type transformations. These utilities are available globally.

\n

Partial<T>

\n

Constructs a type with all properties of T set to optional. This utility will return a type that represents all subsets of a given type.

\n
Example
\n
ts
interface Todo {\n  title: string;\n  description: string;\n}\n\nfunction updateTodo(todo: Todo, fieldsToUpdate: Partial<Todo>) {\n  return { ...todo, ...fieldsToUpdate };\n}\n\nconst todo1 = {\n  title: \"organize desk\",\n  description: \"clear clutter\"\n};\n\nconst todo2 = updateTodo(todo1, {\n  description: \"throw out trash\"\n});
\n

Readonly<T>

\n

Constructs a type with all properties of T set to readonly, meaning the properties of the constructed type cannot be reassigned.

\n
Example
\n
ts
interface Todo {\n  title: string;\n}\n\nconst todo: Readonly<Todo> = {\n  title: \"Delete inactive users\"\n};\n\ntodo.title = \"Hello\"; // Error: cannot reassign a readonly property
\n

This utility is useful for representing assignment expressions that will fail at runtime (i.e. when attempting to reassign properties of a frozen object).

\n
Object.freeze
\n
ts
function freeze<T>(obj: T): Readonly<T>;
\n

Record<K,T>

\n

Constructs a type with a set of properties K of type T. This utility can be used to map the properties of a type to another type.

\n
Example
\n
ts
interface PageInfo {\n  title: string;\n}\n\ntype Page = \"home\" | \"about\" | \"contact\";\n\nconst x: Record<Page, PageInfo> = {\n  about: { title: \"about\" },\n  contact: { title: \"contact\" },\n  home: { title: \"home\" }\n};
\n

Pick<T,K>

\n

Constructs a type by picking the set of properties K from T.

\n
Example
\n
ts
interface Todo {\n  title: string;\n  description: string;\n  completed: boolean;\n}\n\ntype TodoPreview = Pick<Todo, \"title\" | \"completed\">;\n\nconst todo: TodoPreview = {\n  title: \"Clean room\",\n  completed: false\n};
\n

Omit<T,K>

\n

Constructs a type by picking all properties from T and then removing K.

\n
Example
\n
ts
interface Todo {\n  title: string;\n  description: string;\n  completed: boolean;\n}\n\ntype TodoPreview = Omit<Todo, \"description\">;\n\nconst todo: TodoPreview = {\n  title: \"Clean room\",\n  completed: false\n};
\n

Exclude<T,U>

\n

Constructs a type by excluding from T all properties that are assignable to U.

\n
Example
\n
ts
type T0 = Exclude<\"a\" | \"b\" | \"c\", \"a\">; // \"b\" | \"c\"\ntype T1 = Exclude<\"a\" | \"b\" | \"c\", \"a\" | \"b\">; // \"c\"\ntype T2 = Exclude<string | number | (() => void), Function>; // string | number
\n

Extract<T,U>

\n

Constructs a type by extracting from T all properties that are assignable to U.

\n
Example
\n
ts
type T0 = Extract<\"a\" | \"b\" | \"c\", \"a\" | \"f\">; // \"a\"\ntype T1 = Extract<string | number | (() => void), Function>; // () => void
\n

NonNullable<T>

\n

Constructs a type by excluding null and undefined from T.

\n
Example
\n
ts
type T0 = NonNullable<string | number | undefined>; // string | number\ntype T1 = NonNullable<string[] | null | undefined>; // string[]
\n

Parameters<T>

\n

Constructs a tuple type of the types of the parameters of a function type T.

\n
Example
\n
ts
declare function f1(arg: { a: number; b: string }): void;\ntype T0 = Parameters<() => string>; // []\ntype T1 = Parameters<(s: string) => void>; // [string]\ntype T2 = Parameters<<T>(arg: T) => T>; // [unknown]\ntype T4 = Parameters<typeof f1>; // [{ a: number, b: string }]\ntype T5 = Parameters<any>; // unknown[]\ntype T6 = Parameters<never>; // never\ntype T7 = Parameters<string>; // Error\ntype T8 = Parameters<Function>; // Error
\n

ConstructorParameters<T>

\n

The ConstructorParameters<T> type lets us extract all parameter types of a constructor function type. It produces a tuple type with all the parameter types (or the type never if T is not a function).

\n
Example
\n
ts
type T0 = ConstructorParameters<ErrorConstructor>; // [(string | undefined)?]\ntype T1 = ConstructorParameters<FunctionConstructor>; // string[]\ntype T2 = ConstructorParameters<RegExpConstructor>; // [string, (string | undefined)?]
\n

ReturnType<T>

\n

Constructs a type consisting of the return type of function T.

\n
Example
\n
ts
declare function f1(): { a: number; b: string };\ntype T0 = ReturnType<() => string>; // string\ntype T1 = ReturnType<(s: string) => void>; // void\ntype T2 = ReturnType<<T>() => T>; // {}\ntype T3 = ReturnType<<T extends U, U extends number[]>() => T>; // number[]\ntype T4 = ReturnType<typeof f1>; // { a: number, b: string }\ntype T5 = ReturnType<any>; // any\ntype T6 = ReturnType<never>; // any\ntype T7 = ReturnType<string>; // Error\ntype T8 = ReturnType<Function>; // Error
\n

InstanceType<T>

\n

Constructs a type consisting of the instance type of a constructor function type T.

\n
Example
\n
ts
class C {\n  x = 0;\n  y = 0;\n}\n\ntype T0 = InstanceType<typeof C>; // C\ntype T1 = InstanceType<any>; // any\ntype T2 = InstanceType<never>; // any\ntype T3 = InstanceType<string>; // Error\ntype T4 = InstanceType<Function>; // Error
\n

Required<T>

\n

Constructs a type consisting of all properties of T set to required.

\n
Example
\n
ts
interface Props {\n  a?: number;\n  b?: string;\n}\n\nconst obj: Props = { a: 5 }; // OK\n\nconst obj2: Required<Props> = { a: 5 }; // Error: property 'b' missing
\n

ThisParameterType

\n

Extracts the type of the this parameter of a function type, or unknown if the function type has no this parameter.

\n

Note: This type only works correctly if --strictFunctionTypes is enabled. See #32964.

\n
Example
\n
ts
function toHex(this: Number) {\n  return this.toString(16);\n}\n\nfunction numberToString(n: ThisParameterType<typeof toHex>) {\n  return toHex.apply(n);\n}
\n

OmitThisParameter

\n

Removes the ‘this’ parameter from a function type.

\n

Note: This type only works correctly if --strictFunctionTypes is enabled. See #32964.

\n
Example
\n
ts
function toHex(this: Number) {\n  return this.toString(16);\n}\n\n// The return type of `bind` is already using `OmitThisParameter`, this is just for demonstration.\nconst fiveToHex: OmitThisParameter<typeof toHex> = toHex.bind(5);\n\nconsole.log(fiveToHex());
\n

ThisType<T>

\n

This utility does not return a transformed type. Instead, it serves as a marker for a contextual this type. Note that the --noImplicitThis flag must be enabled to use this utility.

\n
Example
\n
ts
// Compile with --noImplicitThis\n\ntype ObjectDescriptor<D, M> = {\n  data?: D;\n  methods?: M & ThisType<D & M>; // Type of 'this' in methods is D & M\n};\n\nfunction makeObject<D, M>(desc: ObjectDescriptor<D, M>): D & M {\n  let data: object = desc.data || {};\n  let methods: object = desc.methods || {};\n  return { ...data, ...methods } as D & M;\n}\n\nlet obj = makeObject({\n  data: { x: 0, y: 0 },\n  methods: {\n    moveBy(dx: number, dy: number) {\n      this.x += dx; // Strongly typed this\n      this.y += dy; // Strongly typed this\n    }\n  }\n});\n\nobj.x = 10;\nobj.y = 20;\nobj.moveBy(5, 5);
\n

In the example above, the methods object in the argument to makeObject has a contextual type that includes ThisType<D & M> and therefore the type of this in methods within the methods object is { x: number, y: number } & { moveBy(dx: number, dy: number): number }. Notice how the type of the methods property simultaneously is an inference target and a source for the this type in methods.

\n

The ThisType<T> marker interface is simply an empty interface declared in lib.d.ts. Beyond being recognized in the contextual type of an object literal, the interface acts like any empty interface.

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