diff --git a/index.d.ts b/index.d.ts
index 150e727..76a858b 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -2,7 +2,7 @@ declare const detectNewline: {
 	/**
 	Detect the dominant newline character of a string.
 
-	@returns Detected newline or `null` when no newline character is found.
+	@returns The detected newline or `undefined` when no newline character is found.
 
 	@example
 	```
@@ -12,7 +12,7 @@ declare const detectNewline: {
 	//=> '\n'
 	```
 	*/
-	(string: string): '\r\n' | '\n' | null;
+	(string: string): '\r\n' | '\n' | undefined;
 
 	/**
 	Detect the dominant newline character of a string.
diff --git a/index.js b/index.js
index e24045a..919598d 100644
--- a/index.js
+++ b/index.js
@@ -1,4 +1,5 @@
 'use strict';
+
 const detectNewline = string => {
 	if (typeof string !== 'string') {
 		throw new TypeError('Expected a string');
@@ -7,10 +8,10 @@ const detectNewline = string => {
 	const newlines = string.match(/(?:\r?\n)/g) || [];
 
 	if (newlines.length === 0) {
-		return null;
+		return;
 	}
 
-	const crlf = newlines.filter(el => el === '\r\n').length;
+	const crlf = newlines.filter(newline => newline === '\r\n').length;
 	const lf = newlines.length - crlf;
 
 	return crlf > lf ? '\r\n' : '\n';
diff --git a/index.test-d.ts b/index.test-d.ts
index 2cc3210..9593f0f 100644
--- a/index.test-d.ts
+++ b/index.test-d.ts
@@ -1,5 +1,5 @@
 import {expectType} from 'tsd';
 import detectNewline = require('.');
 
-expectType<'\r\n' | '\n' | null>(detectNewline('foo\nbar\nbaz\r\n'));
+expectType<'\r\n' | '\n' | undefined>(detectNewline('foo\nbar\nbaz\r\n'));
 expectType<'\r\n' | '\n'>(detectNewline.graceful('foo\nbar\nbaz\r\n'));
diff --git a/readme.md b/readme.md
index c0d2274..f1a1b9a 100644
--- a/readme.md
+++ b/readme.md
@@ -24,11 +24,11 @@ detectNewline('foo\nbar\nbaz\r\n');
 
 ### detectNewline(string)
 
-Returns detected newline or `null` when no newline character is found.
+Returns the detected newline or `undefined` when no newline character is found.
 
 ### detectNewline.graceful(string)
 
-Returns detected newline or `\n` when no newline character is found.
+Returns the detected newline or `\n` when no newline character is found.
 
 
 ## Related
@@ -39,4 +39,4 @@ Returns detected newline or `\n` when no newline character is found.
 
 ## License
 
-MIT © [Sindre Sorhus](http://sindresorhus.com)
+MIT © [Sindre Sorhus](https://sindresorhus.com)
diff --git a/test.js b/test.js
index a09e013..8d0b183 100644
--- a/test.js
+++ b/test.js
@@ -6,6 +6,6 @@ test('main', t => {
 	t.is(detectNewline('foo\r\nbar\r\nbaz\n'), '\r\n');
 	t.is(detectNewline('foo\nbar\nbaz\r\n'), '\n');
 	t.is(detectNewline('foo\nbar\r\n'), '\n');
-	t.is(detectNewline('foo'), null);
+	t.is(detectNewline('foo'), undefined);
 	t.is(detectNewline.graceful('foo'), '\n');
 });