Inference in TypeScript

Where TypeScript works the type out, and how to help it.

function pick<const T extends readonly string[]>(values: T): T[number] {
  return values[0]!;
}

function withDefault<T>(value: T | undefined, fallback: NoInfer<T>): T {
  return value ?? fallback;
}

function pair<A, B = A>(first: A, second: B): [A, B] {
  return [first, second];
}

const chosen = pick(["go", "ts"]); // "go" | "ts", not string
const seconds = withDefault(undefined as number | undefined, 60);
const mixed = pair("a", 1);
const same = pair("a", "b");

console.log(chosen, seconds, mixed[1], same[1]);

How it works

1. Inference flows from the arguments, left to right.
2. A const type parameter keeps literals narrow.
3. NoInfer stops one position driving the inference.

Keywords and builtins used here

• A
• B
• NoInfer
• T
• as
• const
• extends
• function
• number
• readonly
• return
• string

The run, in numbers

Lines

18

Characters to type

513

Tokens

157

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 94 seconds.

Type this snippet

Step 3 of 4 in Type parameters, step 3 of 20 in Generics.

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