性能 - Array.forEach vs实施版本 [英] Performance - Array.forEach vs implemented version

问题描述

什么给出？

http://jsperf.com/native-vs-implmented-0

执行

 函数each（obj，func，context）{ 
 var kindex，
长度; （kindex = 0，length = obj.length; kindex< length; kindex ++）{
 func.call（context，obj [kindex]，kindex，obj）; 
 
 
 code 
 
 $ b 
 测试用例 （$ {
 
 
  //实现了
 each（[0,1,2,3]，function（val）{
 val ++; 
}）
  
 vs  
 
 （$ val $ 
 $ $ $ $ $那么，Array.forEach是如何在内部实现的呢？这是Array.forEach的一个实现。正如你所看到的那样，涉及到许多检查，而不是直接作为你的实现。
 
 
 
请参阅https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Array/forEach  
  if（！Array.prototype.forEach）{
 
 Array.prototype.forEach = function forEach（callback，thisArg）{
 
 var T，k; 
 
 if（this == null）{
 throw new TypeError（this is null or not defined）; 
} 
 
 // 1.设O是调用ToObject传递| this |的结果价值作为论据。 
 var O = Object（this）; 
 
 // 2.让lenValue成为用参数length调用O的Get内部方法的结果。 
 // 3.让len成为ToUint32（lenValue）。 
 var len = O.length>>> 0; // Hack将O.length转换为UInt32 
 
 // 4.如果IsCallable（callback）为false，则引发TypeError异常。 
 //见：http://es5.github.com/#x9.11 
 if（{} .toString.call（callback）！==[object Function]）{
抛出新的TypeError（回调+不是一个函数）; 
} 
 
 // 5.如果提供了这个arg，设T为thisArg;否则让T不确定。 
 if（thisArg）{
 T = thisArg; 
} 
 
 // 6.令k为0 
 k = 0; 
 
 // 7.重复，而k < len 
 while（k< len）{
 
 var kValue; 
 
 // a。让Pk是ToString（k）。 
 //这对于运算符
 // b的LHS操作数是隐含的。让kPresent是用参数Pk调用O的HasProperty内部方法的结果。 
 //这一步可以和c 
 // c结合使用。如果kPresent为真，那么
 if（Object.prototype.hasOwnProperty.call（O，k））{
 
 // i。让kValue是用参数Pk调用O的Get内部方法的结果。 
 kValue = O [k]; 
 
 // ii。使用T作为此值调用Call内部回调方法，
 //包含kValue，k和O的参数列表。
 callback.call（T，kValue，k，O）; 
} 
 // d。将k增加1. 
 k ++; 
} 
 // 8. return undefined 
}; 
} 
  
 
I would have expected a native version to be faster.

What gives?

http://jsperf.com/native-vs-implmented-0

Implementation
function each (obj, func, context) {
    var kindex,
        length;
    for (kindex = 0, length = obj.length; kindex < length; kindex++) {
        func.call(context, obj[kindex], kindex, obj);
    }
}
Test Cases
// implemented
each([0,1,2,3], function(val){
    val++;
})
vs
// native
[0,1,2,3].forEach(function(val){
    val++
})

解决方案
Well, this is how Array.forEach is implemented internally. As you see there are many checks involved and not straight forward as your implementation.

Refer https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Array/forEach
if ( !Array.prototype.forEach ) {
 
  Array.prototype.forEach = function forEach( 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 ( Object.prototype.hasOwnProperty.call(O, k) ) {
 
        // 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
  };
}


                        
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