forEach Polyfill理解

Question

if (!Array.prototype.forEach) {

  Array.prototype.forEach = function(callback/*, thisArg*/) {

    var T, k;

    if (this == null) {
      throw new TypeError('this is null or not defined');
    }

    // 1. Let O be the result of calling toObject() passing the
    // |this| value as the argument.
    var O = Object(this);

    // 2. Let lenValue be the result of calling the Get() internal
    // method of O with the argument "length".
    // 3. Let len be toUint32(lenValue).
    var len = O.length >>> 0;

    // 4. If isCallable(callback) is false, throw a TypeError exception. 
    // See: http://es5.github.com/#x9.11
    if (typeof callback !== 'function') {
      throw new TypeError(callback + ' is not a function');
    }

    // 5. If thisArg was supplied, let T be thisArg; else let
    // T be undefined.
    if (arguments.length > 1) {
      T = arguments[1];
    }

    // 6. Let k be 0.
    k = 0;

    // 7. Repeat while k < len.
    while (k < len) {

      var kValue;

      // a. Let Pk be ToString(k).
      //    This is implicit for LHS operands of the in operator.
      // b. Let kPresent be the result of calling the HasProperty
      //    internal method of O with argument Pk.
      //    This step can be combined with c.
      // c. If kPresent is true, then
      if (k in O) {

        // i. Let kValue be the result of calling the Get internal
        // method of O with argument Pk.
        kValue = O[k];

        // ii. Call the Call internal method of callback with T as
        // the this value and argument list containing kValue, k, and O.
        callback.call(T, kValue, k, O);
      }
      // d. Increase k by 1.
      k++;
    }
    // 8. return undefined.
  };
}

这是一段Polyfill,有些地方不太理解

• var O=Object(this) 为什么要用Object造个O?,下文直接用this[K]访问数组不行?
• O.length >>> 0什么意思?

Answer 1

var O=Object(this)为什么要用Object造个O?

这个是按规范来实现的，可能是为了剔除 undefined 和 null

下文直接用this[K]访问数组不行?

如果 this 是对象的话，O 就是 this

O.length >>> 0什么意思?

右移 0 位，二进制操作返回 32 位整数，限制了 length 范围。数组最大长度是 2^32-1

Answer 2

1. 首先不能保证this一定是一个数组，所以先转化为一个对象以保证可以进行for in循环；
2. 这里位操作符可以将非数字转化为0， 将数字转化为整数，具体可以查阅位操作符中的无符号右移(>>>)的文档

Answer 3

https://stackoverflow.com/que...

https://segmentfault.com/q/10...