string-manipulation

`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.

Definition

type Uppercase<S extends string> = intrinsic

Usage patterns

Pattern	Purpose
Uppercase<'hello'>	Basic uppercase of a literal
Uppercase<`get_${string}`>	Combine with template literal types
Uppercase<T> & string	Ensure result stays assignable to string
as Uppercase<'x'>	Assertion to lock a value's literal case

Examples

type H = Uppercase<'hello'>;

Result: 'HELLO'

type Env = 'dev' | 'prod';
type LoudEnv = Uppercase<Env>;

Distributes over unions: 'DEV' | 'PROD'

type Header<K extends string> = `X-${Uppercase<K>}`;
type H = Header<'id'>; // 'X-ID'

Composing with template literal types

type S = Uppercase<string>;

Result: string (widens when the input is not a literal)

Gotcha

Only affects string LITERAL types — passing a plain `string` widens the result back to `string`. Purely a compile-time transformation, so no runtime toUpperCase call is generated.

Related utility types

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.

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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