TypeScript Academy · Lesson

Default Type Parameters

Provide fallback types when no argument is given.

Lesson 4 of 413 steps

Default Type Parameters is a free TypeScript Academy lesson on CoddyKit. This is lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, and your progress syncs across the web and the CoddyKit app. The TypeScript Academy course includes 4 lessons in total.

Welcome

Default type parameters provide a fallback type when the caller does not specify a type argument. They make generic APIs more ergonomic.

Default Syntax

Add `= DefaultType` after the type parameter name to set a default.

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 type parameters can also have defaults, though they are less common because TypeScript usually infers the type.

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

Defaults with Constraints

A type parameter can have both a constraint and a default. The default must satisfy the constraint.

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

Defaults in React-Like Components

Default type parameters are common in UI library generics for component props.

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

Multiple Defaults

Multiple type parameters can each have 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

If you want to use a default for a later type parameter, all earlier parameters without defaults must be specified.

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

If TypeScript can infer the type argument from context, it uses the inferred type even if a default is provided.

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

Explicit Overrides

Providing an explicit type argument always overrides the default.

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

Defaults in Class Generics

Generic classes can have default type parameters too.

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

Library authors use default type parameters to reduce the boilerplate callers need to write for common use cases.

Quick Check

Given `interface Box`, what is the type of `.value` in `const b: Box = { value: 99 }`?

Answer it in the interactive lesson

Recap

Default type parameters (T = DefaultType) provide fallback types when the caller doesn't specify one. They are most useful in interfaces, class definitions, and library APIs to reduce caller boilerplate.

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Frequently Asked Questions

Is the “Default Type Parameters” lesson free?

Yes — the full text of “Default Type Parameters” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO. The TypeScript Academy course includes 4 lessons in total.

What will I learn in “Default Type Parameters”?

Provide fallback types when no argument is given. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start TypeScript Academy?

No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “Default Type Parameters” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this TypeScript Academy lesson?

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