code: function map(f, a) {
  
    var b =  new Array({});
    for (var i = 0; i < a.length; i++) {
      b[i] = f(typeof a[i] === "string"? a[i]: JSON.stringify(a[i]));
      // JSON.stringnify
      // b[i] = f(a[i]);
    }
    return b;
  }
docs: map the array. Faster than Array.prototype.map
  
  @param  {function} f function
  @param  {Array} a array-like
  @return {Array}   new Array mapped by f