var关键字的目的是什么？我何时应该使用它（或省略它）？ [英] What is the purpose of the var keyword and when should I use it (or omit it)?

问题描述

注意：这个问题是从ECMAScript版本3或5的角度提出的。答案可能会因发布新功能而过时ECMAScript 6。

NOTE: This question was asked from the viewpoint of ECMAScript version 3 or 5. The answers might become outdated with the introduction of new features in the release of ECMAScript 6.

var JavaScript中的关键字，有什么区别

What exactly is the function of the var keyword in JavaScript, and what is the difference between

var someNumber = 2;
var someFunction = function() { doSomething; }
var someObject = { }
var someObject.someProperty = 5;

和

someNumber = 2;
someFunction = function() { doSomething; }
someObject = { }
someObject.someProperty = 5;

？

你什么时候使用一个，为什么/它做什么？

When would you use either one, and why/what does it do?

推荐答案

如果你处于全球范围内，那么差别不大。阅读 Kangax的答案以获得解释

If you're in the global scope then there's not much difference. Read Kangax's answer for explanation

如果您在函数然后 var 将创建一个局部变量，no var将查找范围链，直到找到变量或命中全局范围（此时它将创建它）：

If you're in a function then var will create a local variable, "no var" will look up the scope chain until it finds the variable or hits the global scope (at which point it will create it):

// These are both globals
var foo = 1;
bar = 2;

function()
{
    var foo = 1; // Local
    bar = 2;     // Global

    // Execute an anonymous function
    (function()
    {
        var wibble = 1; // Local
        foo = 2; // Inherits from scope above (creating a closure)
        moo = 3; // Global
    }())
}

如果你没有完成任务，那么你需要使用 var ：

If you're not doing an assignment then you need to use var:

var x; // Declare x

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