js forEach

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本文介绍了JavaScript中forEach方法的两种实现方式，包括一种兼容性实现及Mozilla的实现方案，并通过示例展示了如何使用此方法来遍历数组。

forEach这种方法是一个JavaScript扩展到ECMA-262标准;因此它可能不存在在标准的其他实现。

两种常见的形式

1. http://www.jb51.net/article/67441.htm

<html>
<head>
<title>JavaScript Array forEach Method</title>
</head>
<body>
<script type="text/javascript">
if 
(!Array.prototype.forEach)
{
 Array.prototype.forEach =
function(fun 
/*, thisp*/)
 {
  var
len = this.length;
  if
(typeof 
fun != "function")
   throw
new TypeError();
 
  var
thisp = arguments[1];
  for
(var 
i = 0; i < len; i++)
  {
   if
(i in 
this)
    fun.call(thisp,
this[i], i, 
this);
  }
 };
}
 
function 
printBr(element, index, array) {
 document.write("<br />["
+ index + "] is " 
+ element ); 
}
 
[12, 5, 8, 130, 44].forEach(printBr);
  
</script>
</body>
</html>

这将产生以下结果：

[0] is 12
[1] is 5
[2] is 8
[3] is 130
[4] is 44 

2. mozilla的实现：

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach

// Production steps of ECMA-262, Edition 5, 15.4.4.18
// Reference: http://es5.github.io/#x15.4.4.18
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 = thisArg;
    }

    // 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
  };
}