JavaScript Salesforce ID Check & Expansion

salesforceId.js

/**
 * The regex for a SF ID.
 * @type {RegExp}
 * @const
 */
var ID_REGEX = /^[0-9a-zA-Z]{15}([0-9a-zA-Z]{3})?$/;

/**
 * Array used to expand a SF ID from 15 to 18 characters.
 * @type {Array.<String>}
 * @const
 */
var ID_EXPAND_ARRAY = ['A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z','0','1','2','3','4','5'];

/** 
 * Constructor for SalesforceId object.
 * @param {String} id
 * @this {SalesforceId}
 */
function SalesforceId(id) {
  this.id = id;
}

/**
 * Checks the ID to make sure its format is valid for Salesforce.
 * @this {SalesforceId}
 * @return {Boolean}
 */
SalesforceId.prototype.isValid = function() {
  return ID_REGEX.test(this.id);
};

/**
 * Creates an 18-character ID if not already. If invalid, returns null.
 * @this {SalesforceId}
 * @return {?String}
 */
SalesforceId.prototype.expand = function() {
  if (!this.isValid(this.id)) {
    // Return null if invalid
    return null;

  } else if (this.id.length === 18) {
    // Return as-is if already 18 chars
    return this.id;

  } else {
    var chunks = [this.id.substr(0, 5), this.id.substr(5, 5), this.id.substr(10, 5)]
        , chunkBits = ['','',''];

    for (var i=0;i<chunks[0].length;i++) chunkBits[0] = (/[A-Z]/.test(chunks[0].substr(i, 1)) ? '1' : '0') + chunkBits[0];
    for (var i=0;i<chunks[1].length;i++) chunkBits[1] = (/[A-Z]/.test(chunks[1].substr(i, 1)) ? '1' : '0') + chunkBits[1];
    for (var i=0;i<chunks[2].length;i++) chunkBits[2] = (/[A-Z]/.test(chunks[2].substr(i, 1)) ? '1' : '0') + chunkBits[2];

    chunkBits[0] = parseInt(chunkBits[0], 2);
    chunkBits[1] = parseInt(chunkBits[1], 2);
    chunkBits[2] = parseInt(chunkBits[2], 2);

    return this.id + ID_EXPAND_ARRAY[chunkBits[0]] + ID_EXPAND_ARRAY[chunkBits[1]] + ID_EXPAND_ARRAY[chunkBits[2]];
  }
};