diff --git a/index.d.ts b/index.d.ts
index c5fdb78..3e9dd33 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -1,3 +1,39 @@
+// Source: https://github.com/sindresorhus/type-fest/blob/main/source/internal.d.ts#L49
+type Whitespace =
+	| '\u{9}' // '\t'
+	| '\u{A}' // '\n'
+	| '\u{B}' // '\v'
+	| '\u{C}' // '\f'
+	| '\u{D}' // '\r'
+	| '\u{20}' // ' '
+	| '\u{85}'
+	| '\u{A0}'
+	| '\u{1680}'
+	| '\u{2000}'
+	| '\u{2001}'
+	| '\u{2002}'
+	| '\u{2003}'
+	| '\u{2004}'
+	| '\u{2005}'
+	| '\u{2006}'
+	| '\u{2007}'
+	| '\u{2008}'
+	| '\u{2009}'
+	| '\u{200A}'
+	| '\u{2028}'
+	| '\u{2029}'
+	| '\u{202F}'
+	| '\u{205F}'
+	| '\u{3000}'
+	| '\u{FEFF}';
+
+// Source: https://github.com/sindresorhus/type-fest/blob/main/source/trim.d.ts
+type TrimStart<S extends string> = S extends `${Whitespace}${infer R}` ? TrimStart<R> : S;
+
+type TrimEnd<S extends string> = S extends `${infer R}${Whitespace}` ? TrimEnd<R> : S;
+
+export type Trim<S extends string> = TrimStart<TrimEnd<S>>;
+
 /**
 Trim from the start and end of a string.
 
@@ -9,7 +45,7 @@ trimNewlines('\n🦄\r\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 +58,7 @@ 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 +71,4 @@ trimNewlines.end('\n🦄\r\n');
 //=> '\n🦄'
 ```
 */
-export function trimNewlinesEnd(string: string): string;
+export function trimNewlinesEnd<S extends string>(string: S): TrimEnd<S>;
diff --git a/index.test-d.ts b/index.test-d.ts
index 49b07a1..635c88f 100644
--- a/index.test-d.ts
+++ b/index.test-d.ts
@@ -1,6 +1,16 @@
-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'));
+
+declare const _string: string;
+
+expectType<string>(trimNewlines(_string));
+expectType<string>(trimNewlinesStart(_string));
+expectType<string>(trimNewlinesEnd(_string));