Generate a UID with an optional paramenter to set length. Thanks to http://stackoverflow.com/a/8809472

uid.js

/**
 * Generate a unique ID
 *
 * @param {Number} [len=15] - Optionally set a length of the generated string.
 *                            Defaults to 15 characters, it's not recommended
 *                            to go much lower than this in order to ensure
 *                            uniqueness.
 *
 * @return {string} A unique ID.
 */
function uniqueId (len) {
  var d = new Date().getTime(),
      Xs;

  len = len || 15;
  Xs = new Array(len + 1).join('x');

  return '_' + Xs.replace(/x/g, function (c) {
    var r = (d + Math.random()*16)%16 | 0;
    d = Math.floor(d/16);
    return (c==='x' ? r : (r&0x7|0x8)).toString(16);
  });
};

uid.spec.js

describe("uniqueId", function() {
  it("should generate a UID", function() {
    var UIDs = [],
        last,
        duplicate,
        i;

    // Generate UID's and store them
    for (i = 0; i < 1e4; i++) {
      UIDs.push(uniqueId());
    }

    // Sorting UIDs so we can compare
    UIDs.sort();

    // Set initial array item to compare with
    last = UIDs[0];

    for (i = 1; i < UIDs.length; i++) {

      // Always compare current array item to the previous one
      if (UIDs[i] === last) {
        // If we've done a good job this should never happen
        duplicate = true;
        break;
      }

      last = UIDs[i];
    }

    expect(duplicate).toBeFalsy();
  });

  it("should generate a UID with a minimum length", function() {
    expect(uniqueId()).toMatch(/_\w{15}/);
  });

  it("should generate a UID with a set length", function() {
    var regex;
    for (var i = 1; i <= 20; i++) {
      regex = new RegExp('_\\w{' + i + '}');
      expect(uniqueId(i)).toMatch(regex);
    }
  });
});