TypeScript Utility Types
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
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;
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Required
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
};
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Record
Record is a shortcut to defining an object type with a specific key type and value type.
Record<string, number> is equivalent to { [key: string]: number }
Omit
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
};
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Pick
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
};
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Exclude
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.
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ReturnType
ReturnType extracts the return type of function type.
Example
type PointGenerator = () => { x: number; y: number; };
const point: ReturnType<PointGenerator> = {
x: 10,
y: 20
};
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Parameters
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
};
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Readonly
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.
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