Add isAbsoluteURL helper

lib/helpers/isAbsoluteURL.js

@@ -0,0 +1,14 @@
+'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);
+};