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TypeScript Academy · Lesson

Conditional Types: T extends U ? X : Y

Write type-level if statements with conditional types.

Lesson 3 of 413 steps

Conditional Types: T extends U ? X : Y is a free TypeScript Academy lesson on CoddyKit. This is lesson 3 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

Conditional types let you express type-level if statements. They are a core part of TypeScript's advanced type system used to build utility types.

Basic Syntax

Write T extends U ? X : Y. If T is assignable to U, the result is X; otherwise Y.
type IsString<T> = T extends string ? true : false;
type A = IsString<string>; // true
type B = IsString<number>; // false

NonNullable Built-In

NonNullable is implemented with a conditional type that removes null and undefined from T.
type NonNullable<T> = T extends null | undefined ? never : T;
type C = NonNullable<string | null>; // string

Conditional with Generics

Conditional types are most useful inside generic functions and type aliases.
type Flatten<T> = T extends Array<infer I> ? I : T;
type Str = Flatten<string[]>; // string
type Num = Flatten<number>;   // number

Nested Conditionals

Conditional types can be nested for more complex logic.
type TypeName<T> =
  T extends string ? 'string' :
  T extends number ? 'number' :
  T extends boolean ? 'boolean' :
  'object';

Distribution Over Unions

When T is a bare type parameter, a conditional type distributes over union members.
type ToArray<T> = T extends any ? T[] : never;
type R = ToArray<string | number>; // string[] | number[]

Preventing Distribution

Wrap T in a tuple to prevent distribution.
type NoDistribute<T> = [T] extends [any] ? T[] : never;
type R2 = NoDistribute<string | number>; // (string | number)[]

Using never to Filter

Returning never from a conditional type removes union members.
type OnlyStrings<T> = T extends string ? T : never;
type S = OnlyStrings<'a' | 'b' | 1 | 2>; // 'a' | 'b'

Conditional Types in Mapped Types

Combine mapped types and conditional types to transform object properties selectively.
type NullableValues<T> = { [K in keyof T]: T[K] extends string ? null : T[K] };

Inferring from Function Parameters

Use infer in conditional types to extract parts of complex types.
type FirstParam<T> = T extends (first: infer P, ...rest: any[]) => any ? P : never;
type FP = FirstParam<(a: string, b: number) => void>; // string

Built-In Conditional Utilities

Many built-in utilities (ReturnType, Parameters, InstanceType) are built with conditional types and infer.
type ReturnType<T extends (...args: any) => any> = T extends (...args: any) => infer R ? R : any;

Quick Check

What is the result of `type R = string extends object ? 'yes' : 'no'`?
Answer it in the interactive lesson

Recap

Conditional types (T extends U ? X : Y) enable type-level logic. They distribute over unions, work with infer for extraction, and power many of TypeScript's built-in utility types.
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Frequently Asked Questions

Is the “Conditional Types: T extends U ? X : Y” lesson free?

Yes — the full text of “Conditional Types: T extends U ? X : Y” 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 “Conditional Types: T extends U ? X : Y”?

Write type-level if statements with conditional types. 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 3 of 4.

How long does the “Conditional Types: T extends U ? X : Y” 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?

Yes. Every TypeScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. keyof and Indexed Access Types
  2. Generic Constraints: Narrowing Type Parameters
  3. Conditional Types: T extends U ? X : Y
  4. Distributive Conditional Types
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