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TypeScript comes with a large number of types that can help with some common type manipulation, usually referred to as utility types
Partial changes all the properties in an object to be optional.
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.
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
};
Record<string, number> is equivalent to { [key: string]: number }
Omit removes keys from an object type.
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.
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.
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.
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.
type PointPrinter = (p: { x: number; y: number; }) => void;
const point: Parameters<PointPrinter>[0] = {
x: 10,
y: 20
};