Conditional types and infer in TypeScript

A type-level if, and the keyword that pulls a type out of a position.

type Unwrap<T> = T extends Promise<infer Inner> ? Inner : T;
type ElementOf<T> = T extends readonly (infer Item)[] ? Item : never;
type FirstArg<F> = F extends (first: infer A, ...rest: never[]) => unknown
  ? A
  : never;

type A = Unwrap<Promise<string>>;
type B = ElementOf<number[]>;
type C = FirstArg<(name: string, age: number) => void>;

type Flatten<T> = T extends readonly (infer Item)[] ? Flatten<Item> : T;
type Deep = Flatten<number[][][]>;

const a: A = "unwrapped";
const b: B = 42;
const c: C = "first";
const deep: Deep = 1;
console.log(a, b, c, deep);

How it works

1. A extends B ? X : Y chooses at the type level.
2. infer names a type in the position it appears.
3. That is how ReturnType and Awaited are written.

Keywords and builtins used here

• A
• B
• C
• Deep
• Promise
• T
• const
• extends
• infer
• never
• number
• readonly
• string
• type

The run, in numbers

Lines

18

Characters to type

564

Tokens

181

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 103 seconds.

Type this snippet

Step 2 of 3 in Conditional types, step 2 of 12 in Type-level programming.

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