const Type Parameters (TS 5.0)

TL;DR

Explanation

Without const, TypeScript widens inferred types: function identity<T>(x: T): T with identity(['a','b']) infers T as string[]. With const modifier: function identity<const T>(x: T): T infers T as readonly ['a', 'b']. This enables more precise inference without requiring 'as const' at the call site. Use cases: route definitions, action creators, tuple preservation. The const modifier on a type parameter is similar to inferring with as const. Works with object types too — properties become readonly with literal types.

Common Misconception

Why It Matters

Common Mistakes

• Using const type params when widened types are actually needed.
• Not combining with readonly for tuple preservation.
• Confusing with the const keyword — this is a type-level modifier.

Code Examples

function makeRoute<T extends string>(path: T) { return path; }
const r = makeRoute('/users'); // T inferred as string, not '/users'

function makeRoute<const T extends string>(path: T) { return path; }
const r = makeRoute('/users'); // T inferred as '/users' literal

// With arrays:
function tuple<const T extends unknown[]>(...args: T): T { return args; }
const t = tuple(1, 'two', true); // [1, 'two', true] not (number|string|boolean)[]

References

• ↗ https://www.typescriptlang.org/docs/handbook/release-notes/typescript-5-0.html#const-type-parameters

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