string-manipulation

`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.

Definition

type Uncapitalize<S extends string> = intrinsic

Usage patterns

Pattern	Purpose
Uncapitalize<'Foo'>	Basic first-letter lowercase
`on${Capitalize<E>}`	Event handler naming pattern (paired inverse of Uncapitalize)
Uncapitalize<K & string>	Narrow keyof T to string before transform
Uncapitalize<Capitalize<S>>	Normalize to a guaranteed leading-lowercase form

Examples

type U = Uncapitalize<'Foo'>;

Result: 'foo'

type CamelKeys<T> = { [K in keyof T as Uncapitalize<K & string>]: T[K] };

Rewrites PascalCase keys to camelCase

type Events = 'Click' | 'Focus';
type E = Uncapitalize<Events>;

Result: 'click' | 'focus'

type S = Uncapitalize<string>;

Result: string when input is not a literal

Gotcha

Only mutates the first character — subsequent characters remain untouched, so ALLCAPS becomes aLLCAPS not allcaps. Non-literal inputs widen to `string`.

Related utility 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.

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.

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.

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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