is-relative-url 
Check if an URL is relative
Install
$ npm install --save is-relative-url
Usage
const isRelativeUrl = require('is-relative-url');
isRelativeUrl('foo/bar');
//=> true
isRelativeUrl('http://sindresorhus.com/foo/bar');
//=> false
isRelativeUrl('//sindresorhus.com');
//=> true
Related
See is-absolute-url for the inverse.
License
MIT © Sindre Sorhus