for..in和for循环之间的区别，以及计数器声明 [英] difference between for..in and for loops, and counter declaration

问题描述

for 循环的实现之间的差异(速度，性能，副作用...):

the difference (speed, performace, side effects, ...) between implementations of the for loop:

之间

var i;
for(i = 0; i < length; i++){ //Do something}
// more code

和

for(var i = 0; i < length; i++){ //Do something}
// more code

和

for(i = 0; i < length; i++){ //Do something}
// more code

和之间

var e;
for( e in array){ //Do something}
// more code

和

for(var e in array){ //Do something}
// more code

和

for(e in array){ //Do something}
// more code

推荐答案

没有区别.

JavaScript变量仅具有函数作用域，尽管实际上您可以在for循环的初始化部分中放置var语句，但声明为"

JavaScript variables have only function scope, and although you can place a var statement in the initialisation part of a for loop in reality the declaration is "hoisted" to the top of the scope such that when your second case examples run they're treated exactly like the first cases.

编辑:由于您更新了我回答后的问题 ，因此在不使用var关键字的情况下添加的第三个语法意味着变量(或e)将被创建为全局变量-除非它已经作为全局变量存在，在这种情况下，现有变量将被覆盖.全局变量访问比本地变量访问慢.

Since you updated the question after I answered, the third syntax that you added where you don't use the var keyword means the variable i (or e) will be created as a global - unless it already exists as a global in which case the existing variable will be overwritten. Global variable access is slower than local variable access.

注意:我对这个问题的解释是，它不是将for循环的标准与for..in变量进行比较，而是将不同的变量声明方法相互比较以得出for循环的标准，然后执行相同的操作再次进行for..in循环.

NOTE: my interpretation of the question is that it is not comparing a standard for loop with the for..in variant, it is just compared the different variable declaration methods with each other for a standard for loop, then doing the same again for a for..in loop.

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