javascript - var 关键字的用途是什么以及何时应该使用它(或省略它)？

Question

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 The answers might become outdated with the introduction of new features in the release of ECMAScript 6.

JavaScript中var关键字的作用到底是什么，有什么区别

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

和

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

？

您什么时候会使用其中任何一个，为什么/它有什么作用？

Answer 1

如果您处于全局范围内，则没有太大区别。阅读 Kangax's回答解释

如果你在一个函数中，那么 var 将创建一个局部变量，“no var”将查找作用域链，直到找到该变量或命中全局范围(此时它将创建它):

// 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
    }())
}

如果您不进行作业，则需要使用var:

var x; // Declare x