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

@@ -38,6 +38,7 @@ const slugify = (string, options) => {
 		lowercase: true,
 		decamelize: true,
 		customReplacements: [],
+		preserveLeadingUnderscore: false,
 		...options
 	};
 
@@ -64,9 +65,13 @@ const slugify = (string, options) => {
 		patternSlug = /[^a-z\d]+/g;
 	}
 
+	const shouldPrependUnderscore = options.preserveLeadingUnderscore && string.startsWith('_');
 	string = string.replace(patternSlug, separator);
 	string = string.replace(/\\/g, '');
 	string = removeMootSeparators(string, separator);
+	if (shouldPrependUnderscore) {
+		string = '_' + string;
+	}
 
 	return string;
 };

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

@@ -132,6 +132,23 @@ slugify('foo@unicorn', {
 //=> 'foo-at-unicorn'
 ```
 
+##### preserveLeadingUnderscore
+
+Type: `boolean`\
+Default: `false`
+
+If your string starts with an underscore, it will be preserved in the slugified string.
+
+```js
+const slugify = require('@sindresorhus/slugify');
+
+slugify('_foo_bar');
+//=> 'foo-bar'
+
+slugify('_foo_bar', {preserveLeadingUnderscore: true});
+//=> '_foo-bar'
+```
+
 ## Related
 
 - [slugify-cli](https://github.com/sindresorhus/slugify-cli) - CLI for this module

test.js

@@ -124,3 +124,10 @@ test('supports Turkish', t => {
 test('supports Armenian', t => {
 	t.is(slugify('Ե ր ե ւ ա ն', {lowercase: false, separator: ' '}), 're ye v a n');
 });
+
+test('leading underscore', t => {
+	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');
+	t.is(slugify('____-___foo__bar', {preserveLeadingUnderscore: true}), '_foo-bar');
+});