Add isAbsoluteURL helper

lib/helpers/isAbsoluteURL.js

+'use strict';
+
+/**
+ * Determines whether the specified URL is absolute
+ *
+ * @param {string} url The URL to test
+ * @returns {boolean} True if the specified URL is absolute, otherwise false
+ */
+module.exports = function isAbsoluteURL(url) {
+  // A URL is considered absolute if it begins with "<scheme>://" or "//" (protocol-relative URL).
+  // RFC 3986 defines scheme name as a sequence of characters beginning with a letter and followed
+  // by any combination of letters, digits, plus, period, or hyphen.
+  return /^([a-z][a-z\d\+\-\.]*:)?\/\//i.test(url);
+};

test/specs/helpers/isAbsoluteURL.spec.js

+var isAbsoluteURL = require('../../../lib/helpers/isAbsoluteURL');
+
+describe('helpers::isAbsoluteURL', function () {
+  it('should return true if URL begins with valid scheme name', function () {
+    expect(isAbsoluteURL('https://api.github.com/users')).toBe(true);
+    expect(isAbsoluteURL('custom-scheme-v1.0://example.com/')).toBe(true);
+    expect(isAbsoluteURL('HTTP://example.com/')).toBe(true);
+  });
+
+  it('should return false if URL begins with invalid scheme name', function () {
+    expect(isAbsoluteURL('123://example.com/')).toBe(false);
+    expect(isAbsoluteURL('!valid://example.com/')).toBe(false);
+  });
+
+  it('should return true if URL is protocol-relative', function () {
+    expect(isAbsoluteURL('//example.com/')).toBe(true);
+  });
+
+  it('should return false if URL is relative', function () {
+    expect(isAbsoluteURL('/foo')).toBe(false);
+    expect(isAbsoluteURL('foo')).toBe(false);
+  });
+});