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

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

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
    }())
}

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

var x; // Declare x

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There's a difference.

var x = 1 declares variable x in current scope (aka execution context). If the declaration appears in a function - a local variable is declared; if it's in global scope - a global variable is declared.

x = 1, on the other hand, is merely a property assignment. It first tries to resolve x against scope chain. If it finds it anywhere in that scope chain, it performs assignment; if it doesn't find x, only then does it creates x property on a global object (which is a top level object in a scope chain).

Now, notice that it doesn't declare a global variable, it creates a global property.

The difference between the two is subtle and might be confusing unless you understand that variable declarations also create properties (only on a Variable Object) and that every property in Javascript (well, ECMAScript) have certain flags that describe their properties - ReadOnly, DontEnum and DontDelete.

Since variable declaration creates property with the DontDelete flag, the difference between var x = 1 and x = 1 (when executed in global scope) is that the former one - variable declaration - creates the DontDelete'able property, and latter one doesn't. As a consequence, the property created via this implicit assignment can then be deleted from the global object, and the former one - the one created via variable declaration - cannot be deleted.

But this is just theory of course, and in practice there are even more differences between the two, due to various bugs in implementations (such as those from IE).

Hope it all makes sense : )

[Update 2010/12/16]

In ES5 (ECMAScript 5; recently standardized, 5th edition of the language) there's a so-called "strict mode" — an opt-in language mode, which slightly changes the behavior of undeclared assignments. In strict mode, assignment to an undeclared identifier is a ReferenceError. The rationale for this was to catch accidental assignments, preventing creation of undesired global properties. Some of the newer browsers have already started rolling support for strict mode. See, for example, my compat table.

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Saying it's the difference between "local and global" isn't entirely accurate.

It might be better to think of it as the difference between "local and nearest". The nearest can surely be global, but that won't always be the case.

/* global scope */
var local = true;
var global = true;

function outer() {
    /* local scope */
    var local = true;
    var global = false;

    /* nearest scope = outer */
    local = !global;

    function inner() {
        /* nearest scope = outer */
        local = false;
        global = false;

        /* nearest scope = undefined */
        /* defaults to defining a global */
        public = global;
    }
}

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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

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Always use the var keyword to declare variables. Why? Good coding practice should be enough of a reason in itself, but omitting it means it is declared in the global scope (a variable like this is called an "implied" global). Douglas Crockford recommends never using implied globals, and according to the Apple JavaScript Coding Guidelines:

Any variable created without the var keyword is created at the global scope and is not garbage collected when the function returns (because it doesn’t go out of scope), presenting the opportunity for a memory leak.

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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

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I would say it's better to use var in most situations.

Local variables are always faster than the variables in global scope.

If you do not use var to declare a variable, the variable will be in global scope.

For more information, you can search "scope chain JavaScript" in Google.

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Don't use var!

var was the pre-ES6 way to declare a variable. We are now in the future, and you should be coding as such.

Use const and let

const should be used for ~95% of cases. It makes it so the variable reference can't change, thus array, object, and DOM node properties can change and should likely be const.

let should be be used for any variable expecting to be reassigned. This includes within a for loop. If you ever write varName = beyond the initialization, use let.

Both have block level scoping, as expected in most other languages.

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another difference e.g

var a = a || [] ; // works 

while

a = a || [] ; // a is undefined error.

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Using var is always a good idea to prevent variables from cluttering the global scope and variables from conflicting with each other, causing unwanted overwriting.

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Without var - global variable.

Strongly recommended to ALWAYS use var statement, because init global variable in local context - is evil. But, if you need this dirty trick, you should write comment at start of page:

/* global: varname1, varname2... */

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This is example code I have written for you to understand this concept:

var foo = 5; 
bar = 2;     
fooba = 3;

// Execute an anonymous function
(function() {    
    bar = 100;             //overwrites global scope bar
    var foo = 4;           //a new foo variable is created in this' function's scope
    var fooba = 900;       //same as above
    document.write(foo);   //prints 4
    document.write(bar);   //prints 100
    document.write(fooba); //prints 900
})();

document.write('<br/>');
document.write('<br/>');
document.write(foo);       //prints 5
document.write(bar);       //prints 100
document.write(fooba);     //prints 3

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@Chris S gave a nice example showcasing the practical difference (and danger) between var and no var. Here's another one, I find this one particularly dangerous because the difference is only visible in an asynchronous environment so it can easily slip by during testing.

As you'd expect the following snippet outputs ["text"]:

function var_fun() { let array = [] array.push('text') return array } console.log(var_fun())

So does the following snippet (note the missing let before array):

function var_fun() { array = [] array.push('text') return array } console.log(var_fun())

Executing the data manipulation asynchronously still produces the same result with a single executor:

function var_fun() { array = []; return new Promise(resolve => resolve()).then(() => { array.push('text') return array }) } var_fun().then(result => {console.log(result)})

But behaves differently with multiple ones:

function var_fun() { array = []; return new Promise(resolve => resolve()).then(() => { array.push('text') return array }) } [1,2,3].forEach(i => { var_fun().then(result => {console.log(result)}) })

Using let however:

function var_fun() { let array = []; return new Promise(resolve => resolve()).then(() => { array.push('text') return array }) } [1,2,3].forEach(i => { var_fun().then(result => {console.log(result)}) })

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As someeone trying to learn this this is how I see it. The above examples were maybe a bit overly complicated for a beginner.

If you run this code:

var local = true;
var global = true;


function test(){
  var local = false;
  var global = false;
  console.log(local)
  console.log(global)
}

test();

console.log(local);
console.log(global);

The output will read as: false, false, true, true

Because it sees the variables in the function as seperate from those outside of it, hence the term local variable and this was because we used var in the assignment. If you take away the var in the function so it now reads like this:

var local = true;
var global = true;


function test(){
  local = false;
  global = false;
  console.log(local)
  console.log(global)
}

test();

console.log(local);
console.log(global);

The output is false, false, false, false

This is because rather than creating a new variable in the local scope or function it simply uses the global variables and reassigns them to false.

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I see people are confused when declaring variables with or without var and inside or outside the function. Here is a deep example that will walk you through these steps:

See the script below in action here at jsfiddle

a = 1;// Defined outside the function without var
var b = 1;// Defined outside the function with var
alert("Starting outside of all functions... \n \n a, b defined but c, d not defined yet: \n a:" + a + "\n b:" + b + "\n \n (If I try to show the value of the undefined c or d, console.log would throw 'Uncaught ReferenceError: c is not defined' error and script would stop running!)");

function testVar1(){
    c = 1;// Defined inside the function without var
    var d = 1;// Defined inside the function with var
    alert("Now inside the 1. function: \n a:" + a + "\n b:" + b + "\n c:" + c + "\n d:" + d);

    a = a + 5;
    b = b + 5;
    c = c + 5;
    d = d + 5;

    alert("After added values inside the 1. function: \n a:" + a + "\n b:" + b + "\n c:" + c + "\n d:" + d);
};


testVar1();
alert("Run the 1. function again...");
testVar1();

function testVar2(){
    var d = 1;// Defined inside the function with var
    alert("Now inside the 2. function: \n a:" + a + "\n b:" + b + "\n c:" + c + "\n d:" + d);

    a = a + 5;
    b = b + 5;
    c = c + 5;
    d = d + 5;

    alert("After added values inside the 2. function: \n a:" + a + "\n b:" + b + "\n c:" + c + "\n d:" + d);
};

testVar2();

alert("Now outside of all functions... \n \n Final Values: \n a:" + a + "\n b:" + b + "\n c:" + c + "\n You will not be able to see d here because then the value is requested, console.log would throw error 'Uncaught ReferenceError: d is not defined' and script would stop. \n ");
alert("**************\n Conclusion \n ************** \n \n 1. No matter declared with or without var (like a, b) if they get their value outside the function, they will preserve their value and also any other values that are added inside various functions through the script are preserved.\n 2. If the variable is declared without var inside a function (like c), it will act like the previous rule, it will preserve its value across all functions from now on. Either it got its first value in function testVar1() it still preserves the value and get additional value in function testVar2() \n 3. If the variable is declared with var inside a function only (like d in testVar1 or testVar2) it will will be undefined whenever the function ends. So it will be temporary variable in a function.");
alert("Now check console.log for the error when value d is requested next:");
alert(d);

Conclusion No matter declared with or without var (like a, b) if they get their value outside the function, they will preserve their value and also any other values that are added inside various functions through the script are preserved. If the variable is declared without var inside a function (like c), it will act like the previous rule, it will preserve its value across all functions from now on. Either it got its first value in function testVar1() it still preserves the value and get additional value in function testVar2() If the variable is declared with var inside a function only (like d in testVar1 or testVar2) it will will be undefined whenever the function ends. So it will be temporary variable in a function.

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Inside a code you if you use a variable without using var, then what happens is the automatically var var_name is placed in the global scope eg:

someFunction() {
    var a = some_value; /*a has local scope and it cannot be accessed when this
    function is not active*/
    b = a; /*here it places "var b" at top of script i.e. gives b global scope or
    uses already defined global variable b */
}

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Besides scopes issue, some folks also mention hoisting, but no one gave an example. Here's one for global scope:

console.log(noErrorCase); var noErrorCase = "you will reach that point";

console.log(runTimeError); runTimeError = "you won't reach that point";

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Without using "var" variables can only define when set a value. In example:

my_var;

cannot work in global scope or any other scope. It should be with value like:

my_var = "value";

On the other hand you can define a vaiable like;

var my_var;

Its value is undefined ( Its value is not null and it is not equal to null interestingly.).

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You should use var keyword unless you intend to have the variable attached to window object in browser. Here's a link that explains scoping and difference between glocal scoping and local scoping with and wihtout var keyword.

When variables get defined without the use of var keyword, what it looks like is a simple “assignment” operation.

When the value is assigned to a variable in javascript, the interpreter first tries to find the “variable declaration” in the same context/scope as that of assignment. When the interpreter executes dummyVariable = 20, it looks up for the declaration of dummyVariable at beginning of the function. (Since all Variable declarations are moved to the beginning of the context by javascript interpreter and this is called hoisting)

You may also want to look at hoisting in javascript

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