if (!Array.prototype.map) {  Array.prototype.map = function(callback, thisArg) {    var T, A, k;    if (this == null) {      throw new TypeError(" this is null or not defined");    }    var O = Object(this);    var len = O.length >>> 0;    // 3.如果callback不是函數(shù),則拋出TypeError異常.    if (Object.prototype.toString.call(callback) != "[object Function]") {      throw new TypeError(callback + " is not a function");    }    if (thisArg) {      T = thisArg;    }    A = new Array(len);    k = 0;    while(k < len) {      var kValue, mappedValue;      if (k in O) {        kValue = O[ k ];        mappedValue = callback.call(T, kValue, k, O);        A[ k ] = mappedValue;      }      k++;    }    return A;  };      }var len = O.length >>> 0; 這里位運(yùn)算符目的是什么