Introduction
TypeScript provides several utility types to facilitate common type transformations. These utilities are available globally.
Partial<T>
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.
Example
tsinterface Todo { title: string; description: string; } function updateTodo(todo: Todo, fieldsToUpdate: Partial<Todo>) { return { ...todo, ...fieldsToUpdate }; } const todo1 = { title: "organize desk", description: "clear clutter" }; const todo2 = updateTodo(todo1, { description: "throw out trash" });
Readonly<T>
Constructs a type with all properties of T
set to readonly
, meaning the properties of the constructed type cannot be reassigned.
Example
tsinterface Todo { title: string; } const todo: Readonly<Todo> = { title: "Delete inactive users" }; todo.title = "Hello"; // Error: cannot reassign a readonly property
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
tsfunction freeze<T>(obj: T): Readonly<T>;
Record<K,T>
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.
Example
tsinterface PageInfo { title: string; } type Page = "home" | "about" | "contact"; const x: Record<Page, PageInfo> = { about: { title: "about" }, contact: { title: "contact" }, home: { title: "home" } };
Pick<T,K>
Constructs a type by picking the set of properties K
from T
.
Example
tsinterface Todo { title: string; description: string; completed: boolean; } type TodoPreview = Pick<Todo, "title" | "completed">; const todo: TodoPreview = { title: "Clean room", completed: false };
Omit<T,K>
Constructs a type by picking all properties from T
and then removing K
.
Example
tsinterface Todo { title: string; description: string; completed: boolean; } type TodoPreview = Omit<Todo, "description">; const todo: TodoPreview = { title: "Clean room", completed: false };
Exclude<T,U>
Constructs a type by excluding from T
all properties that are assignable to U
.
Example
tstype T0 = Exclude<"a" | "b" | "c", "a">; // "b" | "c" type T1 = Exclude<"a" | "b" | "c", "a" | "b">; // "c" type T2 = Exclude<string | number | (() => void), Function>; // string | number
Extract<T,U>
Constructs a type by extracting from T
all properties that are assignable to U
.
Example
tstype T0 = Extract<"a" | "b" | "c", "a" | "f">; // "a" type T1 = Extract<string | number | (() => void), Function>; // () => void
NonNullable<T>
Constructs a type by excluding null
and undefined
from T
.
Example
tstype T0 = NonNullable<string | number | undefined>; // string | number type T1 = NonNullable<string[] | null | undefined>; // string[]
Parameters<T>
Constructs a tuple type of the types of the parameters of a function type T
.
Example
tsdeclare function f1(arg: { a: number; b: string }): void; type T0 = Parameters<() => string>; // [] type T1 = Parameters<(s: string) => void>; // [string] type T2 = Parameters<<T>(arg: T) => T>; // [unknown] type T4 = Parameters<typeof f1>; // [{ a: number, b: string }] type T5 = Parameters<any>; // unknown[] type T6 = Parameters<never>; // never type T7 = Parameters<string>; // Error type T8 = Parameters<Function>; // Error
ConstructorParameters<T>
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).
Example
tstype T0 = ConstructorParameters<ErrorConstructor>; // [(string | undefined)?] type T1 = ConstructorParameters<FunctionConstructor>; // string[] type T2 = ConstructorParameters<RegExpConstructor>; // [string, (string | undefined)?]
ReturnType<T>
Constructs a type consisting of the return type of function T
.
Example
tsdeclare function f1(): { a: number; b: string }; type T0 = ReturnType<() => string>; // string type T1 = ReturnType<(s: string) => void>; // void type T2 = ReturnType<<T>() => T>; // {} type T3 = ReturnType<<T extends U, U extends number[]>() => T>; // number[] type T4 = ReturnType<typeof f1>; // { a: number, b: string } type T5 = ReturnType<any>; // any type T6 = ReturnType<never>; // any type T7 = ReturnType<string>; // Error type T8 = ReturnType<Function>; // Error
InstanceType<T>
Constructs a type consisting of the instance type of a constructor function type T
.
Example
tsclass C { x = 0; y = 0; } type T0 = InstanceType<typeof C>; // C type T1 = InstanceType<any>; // any type T2 = InstanceType<never>; // any type T3 = InstanceType<string>; // Error type T4 = InstanceType<Function>; // Error
Required<T>
Constructs a type consisting of all properties of T
set to required.
Example
tsinterface Props { a?: number; b?: string; } const obj: Props = { a: 5 }; // OK const obj2: Required<Props> = { a: 5 }; // Error: property 'b' missing
ThisParameterType
Extracts the type of the this parameter of a function type, or unknown if the function type has no this
parameter.
Example
tsfunction toHex(this: Number) { return this.toString(16); } function numberToString(n: ThisParameterType<typeof toHex>) { return toHex.apply(n); }
OmitThisParameter
Removes the this parameter from a function type.
Example
tsfunction toHex(this: Number) { return this.toString(16); } // The return type of `bind` is already using `OmitThisParameter`, this is just for demonstration. const fiveToHex: OmitThisParameter<typeof toHex> = toHex.bind(5); console.log(fiveToHex());
ThisType<T>
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// Compile with --noImplicitThis type ObjectDescriptor<D, M> = { data?: D; methods?: M & ThisType<D & M>; // Type of 'this' in methods is D & M }; function makeObject<D, M>(desc: ObjectDescriptor<D, M>): D & M { let data: object = desc.data || {}; let methods: object = desc.methods || {}; return { ...data, ...methods } as D & M; } let obj = makeObject({ data: { x: 0, y: 0 }, methods: { moveBy(dx: number, dy: number) { this.x += dx; // Strongly typed this this.y += dy; // Strongly typed this } } }); obj.x = 10; obj.y = 20; obj.moveBy(5, 5);
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.
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.