extractExtension

helpers.js

/**
 * Extract whatever comes after the last '.' in a string.
 *
 * @param {string} string - input path/URL
 * @return {string} chars after last '.', else `undefined`.
 */
var extractExtension = function (string) {
  var ext = string.match(/\.[0-9a-z]+$/i);
  if (ext && ext[0]) {
    return ext[0].replace('.', '').toLowerCase();
  }
  return undefined;
}

hepers.test.js

describe('extractExtension()', function () {

  it('Standard', function () {
    var result = helpers.extractExtension('https://example.com/script.js');
    assert.equal(result, 'js');
  });

  it('Multi-part', function () {
    var result = helpers.extractExtension('https://example.com/script.min.js');
    assert.equal(result, 'js');
  });

  it('None', function () {
    var result1 = helpers.extractExtension('');
    assert.equal(result1, undefined);
    var result2 = helpers.extractExtension('https://example.com/.');
    assert.equal(result2, undefined);
  });

});