Default Type Parameters

Welcome

Default Syntax

interface Container<T = string> {
  value: T;
}
const c: Container = { value: 'hello' }; // T defaults to string
const n: Container<number> = { value: 42 };

Default in Generic Functions

function createList<T = string>(): T[] { return []; }
const list = createList(); // T is string by default

Defaults with Constraints

interface Event<T extends object = Record<string, unknown>> {
  type: string;
  payload: T;
}

Defaults in React-Like Components

interface SelectProps<T = string> {
  options: T[];
  value: T;
  onChange: (val: T) => void;
}

Multiple Defaults

type Pair<A = string, B = number> = { first: A; second: B };
const p: Pair = { first: 'hello', second: 42 }; // A=string, B=number

Trailing Type Parameters Need Defaults Too

type Map<K = string, V = unknown> = Record<K extends string ? K : string, V>;
const m: Map = {}; // K=string, V=unknown by default

When TypeScript Ignores the Default

function wrap<T = string>(val: T): T { return val; }
const n = wrap(42); // T is number, not string (inferred)

Explicit Overrides

interface Box<T = string> { item: T; }
const numBox: Box<number> = { item: 100 }; // T explicitly number

Defaults in Class Generics

class Stack<T = unknown> {
  private items: T[] = [];
  push(item: T) { this.items.push(item); }
  pop(): T | undefined { return this.items.pop(); }
}

Defaults Make Generic Libraries Easier to Use

Quick Check

Recap