TypeScript provides several utility types to facilitate common type transformations. These utilities are available globally.
\nPartial<T>Readonly<T>Record<K,T>Pick<T,K>Omit<T,K>Exclude<T,U>Extract<T,U>NonNullable<T>ReturnType<T>InstanceType<T>Required<T>ThisType<T>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.
\ntsinterface 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});
Readonly<T>Constructs a type with all properties of T set to readonly, meaning the properties of the constructed type cannot be reassigned.
\ntsinterface 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
This utility is useful for representing assignment expressions that will fail at runtime (i.e. when attempting to reassign properties of a frozen object).
\nObject.freeze\ntsfunction 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.
\ntsinterface 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};
Pick<T,K>Constructs a type by picking the set of properties K from T.
\ntsinterface 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};
Omit<T,K>Constructs a type by picking all properties from T and then removing K.
\ntsinterface 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};
Exclude<T,U>Constructs a type by excluding from T all properties that are assignable to U.
\ntstype 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
Extract<T,U>Constructs a type by extracting from T all properties that are assignable to U.
\ntstype T0 = Extract<\"a\" | \"b\" | \"c\", \"a\" | \"f\">; // \"a\"\ntype T1 = Extract<string | number | (() => void), Function>; // () => void
NonNullable<T>Constructs a type by excluding null and undefined from T.
\ntstype T0 = NonNullable<string | number | undefined>; // string | number\ntype T1 = NonNullable<string[] | null | undefined>; // string[]
ReturnType<T>Constructs a type consisting of the return type of function T.
\ntstype 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
InstanceType<T>Constructs a type consisting of the instance type of a constructor function type T.
\ntsclass 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
Required<T>Constructs a type consisting of all properties of T set to required.
\ntsinterface 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
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.
\nts// 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);
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.