Type improvements (#12)

index.d.ts

@@ -1,3 +1,16 @@
+// Source: https://github.com/sindresorhus/type-fest/blob/main/source/internal.d.ts#L49
+type Newline =
+	| '\u{A}' // '\n'
+	| '\u{D}' // '\r'
+	;
+
+// Source: https://github.com/sindresorhus/type-fest/blob/main/source/trim.d.ts
+type TrimStart<S extends string> = S extends `${Newline}${infer R}` ? TrimStart<R> : S;
+
+type TrimEnd<S extends string> = S extends `${infer R}${Newline}` ? TrimEnd<R> : S;
+
+export type Trim<S extends string> = TrimStart<TrimEnd<S>>;
+
 /**
 Trim from the start and end of a string.
 
@@ -9,7 +22,7 @@ trimNewlines('\n🦄\n🦄\r\n');
 //=> '🦄\n🦄'
 ```
 */
-export function trimNewlines(string: string): string;
+export function trimNewlines<S extends string>(string: S): Trim<S>;
 
 /**
 Trim from the start of a string.
@@ -22,7 +35,7 @@ trimNewlines.start('\n🦄\r\n');
 //=> '🦄\r\n'
 ```
 */
-export function trimNewlinesStart(string: string): string;
+export function trimNewlinesStart<S extends string>(string: S): TrimStart<S>;
 
 /**
 Trim from the end of a string.
@@ -35,4 +48,4 @@ trimNewlines.end('\n🦄\r\n');
 //=> '\n🦄'
 ```
 */
-export function trimNewlinesEnd(string: string): string;
+export function trimNewlinesEnd<S extends string>(string: S): TrimEnd<S>;

index.test-d.ts

@@ -1,6 +1,18 @@
-import {expectType} from 'tsd';
+import {expectType, expectAssignable} from 'tsd';
 import {trimNewlines, trimNewlinesStart, trimNewlinesEnd} from './index.js';
 
-expectType<string>(trimNewlines('\n🦄\r\n'));
-expectType<string>(trimNewlinesStart('\n\n🦄\n'));
-expectType<string>(trimNewlinesEnd('\n🦄\n\n'));
+expectAssignable<string>(trimNewlines('\n🦄\r\n'));
+expectAssignable<string>(trimNewlinesStart('\n\n🦄\n'));
+expectAssignable<string>(trimNewlinesEnd('\n🦄\n\n'));
+
+expectType<'🦄'>(trimNewlines('\n🦄\r\n'));
+expectType<'🦄\n'>(trimNewlinesStart('\n\n🦄\n'));
+expectType<'\n🦄'>(trimNewlinesEnd('\n🦄\n\n'));
+
+expectType<' 🦄\n '>(trimNewlines('\n 🦄\n \n'));
+
+declare const _string: string;
+
+expectType<string>(trimNewlines(_string));
+expectType<string>(trimNewlinesStart(_string));
+expectType<string>(trimNewlinesEnd(_string));