Javascript var 关键字的用途是什么，我应该什么时候使用它(或省略它)

发布于09月24日

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 The answers might become outdated with the introduction of new features in the release of ECMAScript 6.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关键字在JavaScript中的具体功能是什么，它们之间的区别是什么

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

和

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

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

推荐答案

如果您在全局范围内，则没有太大区别.阅读Kangax's答案以获得解释

如果您在函数中，则100将创建一个局部变量，"no var"将查找作用域链，直到找到该变量或达到全局作用域(此时它将创建该变量):

// 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
    }())
}

如果你没有做作业(job)，那么你需要使用var: