What is the purpose of the var keyword and when should I use it (or omit it)?

Asked 2009-09-24T08:54:04.117

1665

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 The answers might become outdated with the introduction of new features in the release of ECMAScript 6.

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;

and

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

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

javascript keyword ecmascript-5

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2 Answers

When Javascript is executed in a browser, all your code is surrounded by a with statement, like so:

with (window) {
    //Your code
}

More info on with - MDN

Since var declares a variable in the current scope , there is no difference between declaring var inside window and not declaring it at all.

The difference comes when you're not directly inside the window, e.g. inside a function or inside a block.

Using var lets you hide external variables that have the same name. In this way you can simulate a "private" variable, but that's another topic.

A rule of thumb is to always use var, because otherwise you run the risk of introducing subtle bugs.

EDIT: After the critiques I received, I would like to emphasize the following:

var declares a variable in the current scope
The global scope is window
Not using var implicitly declares var in the global scope (window)
Declaring a variable in the global scope (window) using var is the same as omitting it.
Declaring a variable in scopes different from window using var is not the same thing as declaring a variable without var
Always declare var explicitly because it's good practice

answered 2009-09-24T09:17:57.347

Here's quite a good example of how you can get caught out from not declaring local variables with var:

<script>
one();

function one()
{
    for (i = 0;i < 10;i++)
    {
        two();
        alert(i);
    }
}

function two()
{
    i = 1;
}
</script>

(i is reset at every iteration of the loop, as it's not declared locally in the for loop but globally) eventually resulting in infinite loop

answered 2009-09-24T09:31:30.197

What is the purpose of the var keyword and when should I use it (or omit it)?

2 Answers

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