Question

每个人说for的地方都比forEach更高效例如

Javascript efficiency: 'for' vs 'forEach'

但是在循环中访问当前元素时它是否成立。

如果不是，为什么不，如果是，那么下面的代码呢，这在不同情况下给出了相互矛盾的结果：

逻辑：

//----------------------------------------
var arr = [], len = 1000000, nos = 100;

for(var t = 0; t<len; t++) {
  arr.push({
    a:'a',b:'b',t:Math.random()
  });
}
//----------------------------------------

//===========forEach start====================
var tp1 =  new Date();
arr.forEach( (ik)=>{
  let tmp = [];
  for(let x = 0;x<nos;x++){
    tmp.push(ik.a);
  }
});
var tp2 =  new Date();
//===========forEach end====================


//===========for loop start(let outside loop)====================
var t1 =  new Date();
let i;
for(i = 0; i<arr.length; i++){
  let tmp = [];
  for(let x = 0;x<nos;x++){
    tmp.push(arr[i].a);
  }
}
var t2 =  new Date();
//===========for loop end(let outside loop)====================


//===========for loop start(let inside loop)====================
var ti1 =  new Date();
for(let i = 0; i<arr.length; i++){
  let tmp = [];
  for(let x = 0;x<nos;x++){
    tmp.push(arr[i].a);
  }
}
var ti2 =  new Date();
//===========for loop start(let inside loop)====================


//----------------------------------------

var dff = t2-t1;
var dffi = ti2-ti1;
var dffp = tp2-tp1;

console.log("Execution Time:- (let outside loop): ",dff);
console.log("Execution Time:- (let inside loop): ",dffi);
console.log("Execution Time:- forEach: ",dffp);

console.log("Execution Time:- forEach - for(inside loop): ",dffp-dffi);
console.log("Execution Time:- forEach - for(outside loop): ",dffp-dff);

//----------------------------------------

其中输出如下：

案例1：nos = 100

Execution Time:- (let outside loop):  3739
Execution Time:- (let inside loop):  3699
Execution Time:- forEach:  730
Execution Time:- forEach - for(inside loop):  -2969
Execution Time:- forEach - for(outside loop):  -3009

案例2：nos = 10

Execution Time:- (let outside loop):  427
Execution Time:- (let inside loop):  396
Execution Time:- forEach:  98
Execution Time:- forEach - for(inside loop):  -298
Execution Time:- forEach - for(outside loop):  -329

案例3：nos = 2

Execution Time:- (let outside loop):  106
Execution Time:- (let inside loop):  125
Execution Time:- forEach:  90
Execution Time:- forEach - for(inside loop):  -35
Execution Time:- forEach - for(outside loop):  -16

案例4：nos = 1

Execution Time:- (let outside loop):  62
Execution Time:- (let inside loop):  85
Execution Time:- forEach:  72
Execution Time:- forEach - for(inside loop):  -13
Execution Time:- forEach - for(outside loop):  10

案例5：nos = 0

Execution Time:- (let outside loop):  17
Execution Time:- (let inside loop):  23
Execution Time:- forEach:  72
Execution Time:- forEach - for(inside loop):  49
Execution Time:- forEach - for(outside loop):  55

Answer 1

经过一些分析后，我减少了以下一些事实：

基本上for循环比执行
两者都有一个用于循环的本机（c / c ++）实现。
forEach具有本机（在c / c ++层）实现，用于访问回调中提供的当前元素。

推导

如果我们在迭代时访问当前元素的第三点，forEach将提供更快的结果，因为它将在本机级别执行但是在for循环的情况下它必须传递到两个层{ {1}}然后JS
对于current_item，index，array，native有更高效的变量解析，因为它们是通过本机方法的回调传递的，但是forEach不是

如果我们在循环中访问当前项目，那么/ forEach的性能更高？

1 个答案: