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 # interface 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 # interface 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 # function 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 # interface 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 # interface 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 # interface 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 # type 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 # type 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 # type T0 = NonNullable<string | number | undefined>; string | number type T1 = NonNullable<string[] | null | undefined>; string[] ReturnType<T> # Constructs a type consisting of the return type of function T.

Example # 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 # class 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 # interface Props {

  a?: number;
  b?: string;

};

const obj: Props = { a: 5 }; OK const obj2: Required<Props> = { a: 5 }; Error: property 'b' missing ThisType<T> # This utility does not return a transformed type. Instead, it serves a marker for a contextual this type. Note that the –noImplicitThis flag must be enabled to use this utility.

Example # 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.