Document Uppercase, Lowercase, Capitalize and Uncapitalize bu…

…ilt-in types (#163)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

readme.md

@@ -632,6 +632,90 @@ There are many advanced types most users don't know about.
 	```
 	</details>
 
+- [`Uppercase<S extends string>`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#template-literal-types) - Transforms every character in a string into uppercase.
+	<details>
+	<summary>
+		Example
+	</summary>
+
+	```ts
+	type T = Uppercase<'hello'>;  // 'HELLO'
+
+	type T2 = Uppercase<'foo' | 'bar'>;  // 'FOO' | 'BAR'
+
+	type T3<S extends string> = Uppercase<`aB${S}`>;
+	type T4 = T30<'xYz'>;  // 'ABXYZ'
+
+	type T5 = Uppercase<string>;  // string
+	type T6 = Uppercase<any>;  // any
+	type T7 = Uppercase<never>;  // never
+	type T8 = Uppercase<42>;  // Error, type 'number' does not satisfy the constraint 'string'
+	```
+	</details>
+
+- [`Lowercase<S extends string>`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#template-literal-types) - Transforms every character in a string into lowercase.
+	<details>
+	<summary>
+		Example
+	</summary>
+
+	```ts
+	type T = Lowercase<'HELLO'>;  // 'hello'
+
+	type T2 = Lowercase<'FOO' | 'BAR'>;  // 'foo' | 'bar'
+
+	type T3<S extends string> = Lowercase<`aB${S}`>;
+	type T4 = T32<'xYz'>;  // 'abxyz'
+
+	type T5 = Lowercase<string>;  // string
+	type T6 = Lowercase<any>;  // any
+	type T7 = Lowercase<never>;  // never
+	type T8 = Lowercase<42>;  // Error, type 'number' does not satisfy the constraint 'string'
+	```
+	</details>
+
+- [`Capitalize<S extends string>`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#template-literal-types) - Transforms the first character in a string into uppercase.
+	<details>
+	<summary>
+		Example
+	</summary>
+
+	```ts
+	type T = Capitalize<'hello'>;  // 'Hello'
+
+	type T2 = Capitalize<'foo' | 'bar'>;  // 'Foo' | 'Bar'
+
+	type T3<S extends string> = Capitalize<`aB${S}`>;
+	type T4 = T32<'xYz'>;  // 'ABxYz'
+
+	type T5 = Capitalize<string>;  // string
+	type T6 = Capitalize<any>;  // any
+	type T7 = Capitalize<never>;  // never
+	type T8 = Capitalize<42>;  // Error, type 'number' does not satisfy the constraint 'string'
+	```
+	</details>
+
+- [`Uncapitalize<S extends string>`](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#template-literal-types) - Transforms the first character in a string into lowercase.
+	<details>
+	<summary>
+		Example
+	</summary>
+
+	```ts
+	type T = Uncapitalize<'Hello'>;  // 'hello'
+
+	type T2 = Uncapitalize<'Foo' | 'Bar'>;  // 'foo' | 'bar'
+
+	type T3<S extends string> = Uncapitalize<`AB${S}`>;
+	type T4 = T30<'xYz'>;  // 'aBxYz'
+
+	type T5 = Uncapitalize<string>;  // string
+	type T6 = Uncapitalize<any>;  // any
+	type T7 = Uncapitalize<never>;  // never
+	type T8 = Uncapitalize<42>;  // Error, type 'number' does not satisfy the constraint 'string'
+	```
+	</details>
+
 You can find some examples in the [TypeScript docs](https://www.typescriptlang.org/docs/handbook/advanced-types.html#predefined-conditional-types).
 
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