Loading lesson path
TypeScript comes with a large number of types that can help with some common type manipulation, usually referred to as utility types. This chapter covers the most popular utility types.
Partial changes all the properties in an object to be optional.
Example interface Point {
x: number;
y: number;
}
let pointPart: Partial<Point> = {}; // `Partial` allows x and y to be optional pointPart.x = 10;Required changes all the properties in an object to be required.
Example interface Car {
make: string;
model: string;
mileage?: number;
}
let myCar: Required<Car> = {
make: 'Ford', model: 'Focus', mileage: 12000 // `Required` forces mileage to be defined
};Record is a shortcut to defining an object type with a specific key type and value type.
const nameAgeMap: Record<string, number> = {
'Alice': 21,
'Bob': 25
};Formula
Record < string, number >is equivalent to
{ [key: string]: number }Omit removes keys from an object type.
Example interface Person {
name: string;
age: number;
location?: string;
}
const bob: Omit<Person, 'age' | 'location'> = {
name: 'Bob'
// `Omit` has removed age and location from the type and they can't be defined here
};Pick removes all but the specified keys from an object type.
Example interface Person {
name: string;
age: number;
location?: string;
}
const bob: Pick<Person, 'name'> = {
name: 'Bob'
// `Pick` has only kept name, so age and location were removed from the type and they can't be defined here
};Exclude removes types from a union.
Example type Primitive = string | number | boolean const value: Exclude<Primitive, string> = true; // a string cannot be used here since Exclude removed it from the type.
ReturnType extracts the return type of a function type.Example type PointGenerator = () => { x: number; y: number; };
const point: ReturnType<PointGenerator> = {x: 10, y: 20
};Parameters extracts the parameter types of a function type as an array.
Example type PointPrinter = (p: { x: number; y: number; }) => void;
const point: Parameters<PointPrinter>[0] = {x: 10, y: 20
};Readonly is used to create a new type where all properties are readonly, meaning they cannot be modified once assigned a value. Keep in mind TypeScript will prevent this at compile time, but in theory since it is compiled down to JavaScript you can still override a readonly property.
Example interface Person {
name: string;
age: number;
}
const person: Readonly<Person> = {
name: "Dylan", age: 35,
};
person.name = 'Israel'; // prog.ts(11,8): error TS2540: Cannot assign to 'name' because it is a read-only property.