Add KebabCase<>

As dicussed in TypeScript 4.1 type ideas sindresorhus#134

Author: voxpelli

index.d.ts

@@ -10,6 +10,7 @@ export {RequireAtLeastOne} from './source/require-at-least-one';
 export {RequireExactlyOne} from './source/require-exactly-one';
 export {PartialDeep} from './source/partial-deep';
 export {ReadonlyDeep} from './source/readonly-deep';
+export {KebabCase} from './source/kebab-case';
 export {LiteralUnion} from './source/literal-union';
 export {Promisable} from './source/promisable';
 export {Opaque} from './source/opaque';

source/kebab-case.d.ts

@@ -0,0 +1,51 @@
+type SplitIncludingDelimitor<S extends string, D extends string> =
+    string extends S ? string[] :
+    S extends '' ? [] :
+    S extends `${infer T}${D}${infer U}` ?
+    ( S extends `${T}${infer Z}${U}` ? [T, Z, ...SplitIncludingDelimitor<U, D>] : never ) :
+    [S];
+
+type Join<S extends any[], D extends string> =
+    string[] extends S ? string :
+    S extends [`${infer T}`, ...infer U] ?
+    U[0] extends undefined ? T : `${T}${D}${Join<U, D>}` :
+    '';
+
+type UpperCaseChars = 'A' | 'B' | 'C' | 'D' | 'E' | 'F' | 'G' | 'H' | 'I' | 'J' | 'K' | 'L' | 'M' | 'N' | 'O' | 'P' | 'Q' | 'R' | 'S' | 'T' | 'U' | 'V' | 'X' | 'Y' | 'Z';
+type WordSeparators = "-"|"_"|" ";
+
+type StringPartToKebabCase<K extends string, S extends string, L extends string> =
+    K extends S ? '-' :
+    K extends L ? `-${Lowercase<K>}` :
+    K;
+type StringArrayToKebabCase<K extends any[], S extends string, L extends string> =
+    K extends [`${infer T}`, ...infer U] ? `${StringPartToKebabCase<T, S, L>}${StringArrayToKebabCase<U, S, L>}` :
+		'';
+
+/**
+Converts a string literal that may use another casing than kebab casing to kebab casing
+
+This can be useful when eg. converting a camel cased object property to eg. a CSS class name or a CLI option.
+
+@example
+```
+import {KebabCase} from 'type-fest';
+
+type KebabCasedProps<T> = {
+    [K in keyof T as KebabCase<K>]: T[K]
+};
+
+interface CliOptions {
+		dryRun: boolean;
+		includeFile: string;
+    foo: number;
+}
+
+const rawCliOptions: KebabCasedProps<CliOptions> = {
+		'dry-run': true,
+		'include-file': 'bar.js',
+    foo: 123
+};
+```
+*/
+export type KebabCase<K> = K extends string ? StringArrayToKebabCase<SplitIncludingDelimitor<K, WordSeparators | UpperCaseChars>, WordSeparators, UpperCaseChars> : K;

test-d/kebab-case.ts

@@ -0,0 +1,34 @@
+import {KebabCase} from '../source/kebab-case';
+import {expectType, expectAssignable} from 'tsd';
+
+const kebabFromCamel: KebabCase<'fooBar'> = 'foo-bar';
+expectType<'foo-bar'>(kebabFromCamel);
+
+const kebabFromKebab: KebabCase<'foo-bar'> = 'foo-bar';
+expectType<'foo-bar'>(kebabFromKebab);
+
+const kebabFromSpace: KebabCase<'foo bar'> = 'foo-bar';
+expectType<'foo-bar'>(kebabFromSpace);
+
+const kebabFromSnake: KebabCase<'foo_bar'> = 'foo-bar';
+expectType<'foo-bar'>(kebabFromSnake);
+
+const noKebabFromMono: KebabCase<'foobar'> = 'foobar';
+expectType<'foobar'>(noKebabFromMono);
+
+// Verifying example
+type KebabCasedProps<T> = {
+    [K in keyof T as KebabCase<K>]: T[K]
+};
+
+interface CliOptions {
+		dryRun: boolean;
+		includeFile: string;
+    foo: number;
+}
+
+expectAssignable<KebabCasedProps<CliOptions>>({
+		'dry-run': true,
+		'include-file': 'bar.js',
+    foo: 123
+});