TypeScript Required<T> Utility Type Explained

Understanding Required

Required<T> constructs a type with all properties of T set to required. It is the inverse of Partial<T>, removing the optional modifier (?) from every property.

Syntax

type Required<T> = {
  [P in keyof T]-?: T[P];
};

The -? syntax removes the optional modifier. This is a lesser-known TypeScript feature called a "mapping modifier removal."

Basic Example

interface Config {
  host?: string;
  port?: number;
  debug?: boolean;
  timeout?: number;
}

type StrictConfig = Required<Config>;
// Equivalent to:
// {
//   host: string;
//   port: number;
//   debug: boolean;
//   timeout: number;
// }

Practical Pattern: Validated Configuration

A common pattern is accepting a partial config from the user, applying defaults, and then treating the result as a fully required type:

const defaults: Required<Config> = {
  host: "localhost",
  port: 3000,
  debug: false,
  timeout: 5000,
};

function createApp(userConfig: Config): App {
  const config: Required<Config> = { ...defaults, ...userConfig };
  // All fields are guaranteed to exist
  console.log(config.host); // string, not string | undefined
  return new App(config);
}

Combining with Pick

You can use Required with Pick to make only specific properties required:

type WithRequiredId = Partial<User> & Required<Pick<User, "id">>;
// id is required, everything else is optional