Add leading underscore option

index.d.ts

@@ -80,6 +80,24 @@ declare namespace slugify {
 		```
 		*/
 		readonly customReplacements?: ReadonlyArray<[string, string]>;
+
+		/**
+		It preserves leading underscore.
+
+		@default false
+
+		@example
+		```
+		import slugify = require('@sindresorhus/slugify');
+
+		slugify('_foo_bar');
+		//=> 'foo-bar'
+
+		slugify('_foo_bar', {preserveLeadingUnderscore: true});
+		//=> '_foo-bar'
+		```
+		*/
+		readonly preserveLeadingUnderscore?: boolean;
 	}
 }
 

index.js

@@ -35,10 +35,10 @@ const slugify = (string, options) => {
 
 	options = {
 		separator: '-',
-		leadingUnderscore: false,
 		lowercase: true,
 		decamelize: true,
 		customReplacements: [],
+		preserveLeadingUnderscore: false,
 		...options
 	};
 
@@ -65,7 +65,7 @@ const slugify = (string, options) => {
 		patternSlug = /[^a-z\d]+/g;
 	}
 
-	if (options.leadingUnderscore) {
+	if (options.preserveLeadingUnderscore) {
 		let patternSlugString = patternSlug.source;
 		patternSlugString = `(?!^)_?${patternSlugString}`;
 		patternSlug = new RegExp(patternSlugString, 'g');

index.test-d.ts

@@ -8,3 +8,4 @@ expectType<string>(slugify('fooBar', {decamelize: false}));
 expectType<string>(
 	slugify('I ♥ 🦄 & 🐶', {customReplacements: [['🐶', 'dog']]})
 );
+expectType<string>(slugify('_foo_bar', {preserveLeadingUnderscore: true}));

readme.md

@@ -132,20 +132,20 @@ slugify('foo@unicorn', {
 //=> 'foo-at-unicorn'
 ```
 
-##### customReplacements
+##### preserveLeadingUnderscore
 
 Type: `boolean`\
 Default: `false`
 
-Allow leading underscore as an escape `_foo_bar` → `_foo-bar`.
+It preserves leading underscore: `_foo_bar` → `_foo-bar`.
 
 ```js
 const slugify = require('@sindresorhus/slugify');
 
 slugify('_foo_bar');
 //=> 'foo-bar'
 
-slugify('_foo_bar', {leadingUnderscore: true});
+slugify('_foo_bar', {preserveLeadingUnderscore: true});
 //=> '_foo-bar'
 ```
 

test.js

@@ -126,6 +126,7 @@ test('supports Armenian', t => {
 });
 
 test('leading underscore', t => {
-	t.is(slugify('_foo bar', {leadingUnderscore: true}), '_foo-bar');
-	t.is(slugify('_foo_bar', {leadingUnderscore: true}), '_foo-bar');
+	t.is(slugify('_foo bar', {preserveLeadingUnderscore: true}), '_foo-bar');
+	t.is(slugify('_foo_bar', {preserveLeadingUnderscore: true}), '_foo-bar');
+	t.is(slugify('__foo__bar', {preserveLeadingUnderscore: true}), '_-foo-bar');
 });