Utility Types

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

Awaited<Type>

Released: 4.5

This type is meant to model operations like await in async functions, or the .then() method on Promises - specifically, the way that they recursively unwrap Promises.

Example

ts
type = <<string>>;
type A = string
type = <<<number>>>;
type B = number
type = <boolean | <number>>;
type C = number | booleanTry

Partial<Type>

Released:
2.1

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

Example

ts
interface {
: string;
: string;
}
function(: , : <>) {
return { ..., ... };
}
const = {
:"organize desk",
:"clear clutter",
};
const = (, {
:"throw out trash",
});Try

Required<Type>

Released:
2.8

Constructs a type consisting of all properties of Type set to required. The opposite of Partial.

Example

ts
interface {
?: number;
?: string;
}
const:  = { :5 };
const: <> = { :5 };
Property 'b' is missing in type '{ a: number; }' but required in type 'Required<Props>'.2741Property 'b' is missing in type '{ a: number; }' but required in type 'Required<Props>'.Try

Readonly<Type>

Released:
2.1

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

Example

ts
interface {
: string;
}
const: <> = {
:"Delete inactive users",
};
. = "Hello";
Cannot assign to 'title' because it is a read-only property.2540Cannot assign to 'title' because it is a read-only property.Try

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

Object.freeze

ts
functionfreeze<Type>(obj: Type): Readonly<Type>;

Record<Keys, Type>

Released:
2.1

Constructs an object type whose property keys are Keys and whose property values are Type. This utility can be used to map the properties of a type to another type.

Example

ts
type = "miffy" | "boris" | "mordred";
interface {
: number;
: string;
}
const: <, > = {
: { :10, :"Persian" },
: { :5, :"Maine Coon" },
: { :16, :"British Shorthair" },
};
.;
const cats: Record<CatName, CatInfo>Try

Pick<Type, Keys>

Released:
2.1

Constructs a type by picking the set of properties Keys (string literal or union of string literals) from Type.

Example

ts
interface {
: string;
: string;
: boolean;
}
type = <, "title" | "completed">;
const:  = {
:"Clean room",
:false,
};
;
const todo: TodoPreviewTry

Omit<Type, Keys>

Released:
3.5

Constructs a type by picking all properties from Type and then removing Keys (string literal or union of string literals). The opposite of Pick.

Example

ts
interface {
: string;
: string;
: boolean;
: number;
}
type = <, "description">;
const:  = {
:"Clean room",
:false,
:1615544252770,
};
;
const todo: TodoPreview
type = <, "completed" | "createdAt">;
const:  = {
:"Pick up kids",
:"Kindergarten closes at 5pm",
};
;
const todoInfo: TodoInfoTry

Exclude<UnionType, ExcludedMembers>

Released:
2.8

Constructs a type by excluding from UnionType all union members that are assignable to ExcludedMembers.

Example

ts
type = <"a" | "b" | "c", "a">;
type T0 = "b" | "c"
type = <"a" | "b" | "c", "a" | "b">;
type T1 = "c"
type = <string | number | (() =>void), >;
type T2 = string | number
type =
 | { : "circle"; : number }
 | { : "square"; : number }
 | { : "triangle"; : number; : number };
type = <, { : "circle" }>
type T3 = {
 kind: "square";
 x: number;
} | {
 kind: "triangle";
 x: number;
 y: number;
}Try

Extract<Type, Union>

Released:
2.8

Constructs a type by extracting from Type all union members that are assignable to Union.

Example

ts
type = <"a" | "b" | "c", "a" | "f">;
type T0 = "a"
type = <string | number | (() =>void), >;
type T1 = () => void
type =
 | { : "circle"; : number }
 | { : "square"; : number }
 | { : "triangle"; : number; : number };
type = <, { : "circle" }>
type T2 = {
 kind: "circle";
 radius: number;
}Try

NonNullable<Type>

Released:
2.8

Constructs a type by excluding null and undefined from Type.

Example

ts
type = <string | number | undefined>;
type T0 = string | number
type = <string[] | null | undefined>;
type T1 = string[]Try

Parameters<Type>

Released:
3.1

Constructs a tuple type from the types used in the parameters of a function type Type.

For overloaded functions, this will be the parameters of the last signature; see Inferring Within Conditional Types.

Example

ts
declarefunction(: { : number; : string }): void;
type = <() =>string>;
type T0 = []
type = <(: string) =>void>;
type T1 = [s: string]
type = <<>(: ) =>>;
type T2 = [arg: unknown]
type = <typeof>;
type T3 = [arg: {
 a: number;
 b: string;
}]
type = <any>;
type T4 = unknown[]
type = <never>;
type T5 = never
type = <>;
Type 'string' does not satisfy the constraint '(...args: any) => any'.2344Type 'string' does not satisfy the constraint '(...args: any) => any'.
type T6 = never
type = <>;
Type 'Function' does not satisfy the constraint '(...args: any) => any'.
 Type 'Function' provides no match for the signature '(...args: any): any'.2344Type 'Function' does not satisfy the constraint '(...args: any) => any'.
 Type 'Function' provides no match for the signature '(...args: any): any'.
type T7 = neverTry

ConstructorParameters<Type>

Released:
3.1

Constructs a tuple or array type from the types of a constructor function type. It produces a tuple type with all the parameter types (or the type never if Type is not a function).

Example

ts
type = <>;
type T0 = [message?: string]
type = <>;
type T1 = string[]
type = <>;
type T2 = [pattern: string | RegExp, flags?: string]
class {
constructor(: number, : string) {}
}
type = <typeof>;
type T3 = [a: number, b: string]
type = <any>;
type T4 = unknown[]
type = <>;
Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
 Type 'Function' provides no match for the signature 'new (...args: any): any'.2344Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
 Type 'Function' provides no match for the signature 'new (...args: any): any'.
type T5 = neverTry

ReturnType<Type>

Released:
2.8

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

For overloaded functions, this will be the return type of the last signature; see Inferring Within Conditional Types.

Example

ts
declarefunction(): { : number; : string };
type = <() =>string>;
type T0 = string
type = <(: string) =>void>;
type T1 = void
type = <<>() =>>;
type T2 = unknown
type = <<extends, extendsnumber[]>() =>>;
type T3 = number[]
type = <typeof>;
type T4 = {
 a: number;
 b: string;
}
type = <any>;
type T5 = any
type = <never>;
type T6 = never
type = <>;
Type 'string' does not satisfy the constraint '(...args: any) => any'.2344Type 'string' does not satisfy the constraint '(...args: any) => any'.
type T7 = any
type = <>;
Type 'Function' does not satisfy the constraint '(...args: any) => any'.
 Type 'Function' provides no match for the signature '(...args: any): any'.2344Type 'Function' does not satisfy the constraint '(...args: any) => any'.
 Type 'Function' provides no match for the signature '(...args: any): any'.
type T8 = anyTry

InstanceType<Type>

Released:
2.8

Constructs a type consisting of the instance type of a constructor function in Type.

Example

ts
class {
 = 0;
 = 0;
}
type = <typeof>;
type T0 = C
type = <any>;
type T1 = any
type = <never>;
type T2 = never
type = <>;
Type 'string' does not satisfy the constraint 'abstract new (...args: any) => any'.2344Type 'string' does not satisfy the constraint 'abstract new (...args: any) => any'.
type T3 = any
type = <>;
Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
 Type 'Function' provides no match for the signature 'new (...args: any): any'.2344Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
 Type 'Function' provides no match for the signature 'new (...args: any): any'.
type T4 = anyTry

NoInfer<Type>

Released:
5.4

Blocks inferences to the contained type. Other than blocking inferences, NoInfer<Type> is identical to Type.

Example

ts
functioncreateStreetLight<Cextendsstring>(
colors: C[],
defaultColor?: NoInfer<C>,
) {
// ...
}
createStreetLight(["red", "yellow", "green"], "red"); // OK
createStreetLight(["red", "yellow", "green"], "blue"); // Error

ThisParameterType<Type>

Released:
3.3

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

Example

ts
function(: ) {
returnthis.(16);
}
function(: <typeof>) {
return.();
}Try

OmitThisParameter<Type>

Released:
3.3

Removes the this parameter from Type. If Type has no explicitly declared this parameter, the result is simply Type. Otherwise, a new function type with no this parameter is created from Type. Generics are erased and only the last overload signature is propagated into the new function type.

Example

ts
function(: ) {
returnthis.(16);
}
const: <typeof> = .(5);
.(());Try

ThisType<Type>

Released:
2.3

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.

Example

ts
type<, > = {
?: ;
?:  & < & >; // Type of 'this' in methods is D & M
};
function<, >(: <, >):  &  {
let: object = . || {};
let: object = . || {};
return { ..., ... } as & ;
}
let = ({
: { :0, :0 },
: {
(: number, : number) {
this. += ; // Strongly typed this
this. += ; // Strongly typed this
 },
 },
});
. = 10;
. = 20;
.(5, 5);Try

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): void }. Notice how the type of the methods property simultaneously is an inference target and a source for the this type in methods.

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.

Intrinsic String Manipulation Types

Uppercase<StringType>

Lowercase<StringType>

Capitalize<StringType>

Uncapitalize<StringType>

To help with string manipulation around template string literals, TypeScript includes a set of types which can be used in string manipulation within the type system. You can find those in the Template Literal Types documentation.