javascript - var关键字的目的是什么？何时应使用（或省略）？(What is the purpose of the var keyword and when should I use it (or omit it)?)

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 The answers might become outdated with the introduction of new features in the release of ECMAScript 6.

(注意 ：从ECMAScript版本3或5的角度提出了此问题。随着ECMAScript 6版本中引入新功能，答案可能会过时。)

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

(JavaScript中var关键字的功能到底是什么，它们之间有什么区别)

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

and

(和)

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

?

(？)

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

(您什么时候会使用其中一个？为什么/会做什么？)

  ask by Alex translate from so

Answer 1

If you're in the global scope then there's not much difference.

(如果您位于全球范围内，则没有太大区别。)

Read Kangax's answer for explanation

(阅读Kangax的答案以获取解释)

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):

(如果您在函数中，则var将创建一个局部变量，“ no var”将查找作用域链，直到找到该变量或达到全局作用域为止（此时将创建它）：)

// 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
    }())
}

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

(如果您不进行分配，则需要使用var ：)

var x; // Declare x