Utility Types

Types built into TypeScript that help with common type manipulation.

Partial

Makes all properties in an object optional.

interface Coords {
  x: number;
  y: number;
}

let coordsPart: Partial<Coords> = {}; // `Partial` allows x
// and y to be optional
coordsPart.x = 10;

Required

Makes all properties in an object required.

interface Vehicle {
  make: string;
  model: string;
  mileage?: number;
}

let myCar: Required<Vehicle> = {
  make: "Honda",
  model: "Civic",
  mileage: 30000, // `Required` forces mileage to be defined
};

Record

Creates an object type whose property keys are Keys and whose property values are Type.

const ageNameMap: Record<string, number> = {
  Matt: 20,
  Michael: 24,
};
//`Record<string, number>` is the same as `{ [key: string]: number }`

Omit

Remove specific keys from an object type:

interface Employee {
  name: string;
  age: number;
  location?: string;
}

// Only name is allowed - age and location are omitted
const matt: Omit<Employee, "age" | "location"> = {
  name: "Matt",
};

Pick

Keeps only the specified keys (opposite of Omit).

// Only name is allowed - everything else is excluded
const michael: Pick<Employee, "name"> = {
  name: "Michael",
};

Exclude

Removes types from a union.

type Primitive = string | number | boolean;
const value3: Exclude<Primitive, string> = true; // a string cannot
// be used since Exclude removed it from the type.

Return type

Extracts return type from function type.

type CoordGenerator = () => { x: number; y: number };
const coord: ReturnType<CoordGenerator> = {
  x: 10,
  y: 20,
};

Parameters

Extracts the parameters types from a function (as an array).

type CoordPrinter = (p: { x: number; y: number }) => void;
const coord2: Parameters<CoordPrinter>[0] = {
  x: 10,
  y: 20,
};

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