JavaScript1.6新特性系列之forEach(翻译)

JavaScript1.6新特性系列之 forEach 

 

总结：数组中的每一项多按照指定的方法执行。

 

Method of Array

Implemented in	JavaScript 1.6
ECMAScript Edition	ECMAScript 5th Edition

 

 

语法：

 

array.forEach(callback[,thisArg]);

 

参数：

 

• callback  ---每一项指定的执行方法
• thisArg   ---当callback方法执行时候的this对象

通用性：

     forEach 是加入ECMA-262标准里面的，可能在别的标准里面没有它。下面的代码你可以放在你的脚本前面，它可以支持你使用forEach.

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; // Hack to convert O.length to a UInt32

    // 4. If IsCallable(callback) is false, throw a TypeError exception.
    // See: http://es5.github.com/#x9.11
    if ( {}.toString.call(callback) != "[object Function]" ) {
      throw new TypeError( callback + " is not a function" );
    }

    // 5. If thisArg was supplied, let T be thisArg; else let T be undefined.
    if ( thisArg ) {
      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
  };
}

 
 

举例：

[10,20,30,40].forEach(function(elem,index){
   console.log('index='+index+"&elem="+elem);
});

//输出
index=0&elem=10
index=1&elem=20
index=2&elem=30
index=3&elem=40

 

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