string-manipulation

`Lowercase<S>`

Intrinsic string type manipulator that converts each character of literal S to lowercase. Available since TS 4.1 and evaluated by the compiler with no runtime cost.

Definition

type Lowercase<S extends string> = intrinsic

Usage patterns

Pattern	Purpose
Lowercase<'HELLO'>	Basic lowercase transform
Lowercase<HTTPMethod>	Normalize a union of literal cases
Lowercase<S> extends S ? true : false	Test whether a literal is already lowercase
Uppercase<Lowercase<S>>	Two-step normalize/loudify

Examples

type L = Lowercase<'WORLD'>;

Result: 'world'

type HTTPMethod = 'GET' | 'POST' | 'PUT';
type Method = Lowercase<HTTPMethod>;

Result: 'get' | 'post' | 'put' — distributes over unions

type Path<T extends string> = `/${Lowercase<T>}`;
type P = Path<'API'>; // '/api'

Building URL patterns via template literal types

type S = Lowercase<string>;

Result: string when the input is not a literal

Gotcha

Only literal string types are transformed — a bare `string` stays `string`. Emits no runtime code, so pair with String.prototype.toLowerCase() for actual value conversion.

Related utility types

Uppercase<S>

Compile-time intrinsic that converts a string literal type S to its uppercase form. Introduced in TS 4.1 alongside template literal types.

Capitalize<S>

Intrinsic string type utility that uppercases the first character of literal string S, leaving the rest unchanged. Enables typed camelCase-to-PascalCase and getter-name construction.

Uncapitalize<S>

Intrinsic type utility that lowercases the first character of literal S, leaving the rest unchanged. The inverse of Capitalize and widely used for PascalCase-to-camelCase mappings.

Record<K, V>

Creates an object type whose keys come from the union K and whose values all have type V. Useful for lookup tables, dictionaries, and enum-keyed maps.

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