javascript variable

do we have to declare js variable like this:

var x=5;

or just simple like this

x=5;
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what is the differences?... will it effect the functionality of the variable?...

variable_name = 5 would always put the variable_name into the global object (which is window in a browser)

If you are in the global context (= not a function context) the two statements are basically the same. But if you are in a function context, var makes sure that this variable is only declared within the current context.

So for instance:

function foobar() {
    bar = 55;
}

foobar();

window.bar === 55 // true

Better:

function foobar() {
    var bar = 55;
}

window.bar === 55 // false

Conclusion: always use var within the context of a function. This avoids clobbering / overriding the global object with variables.

The var keyword limits the scope of the variable to the current function. Leaving it off makes a global. Globals are bad and should be avoided as they are a key source of race conditions and scripts interfering with each other.

x=5 means this variable (after execution) will become global and can be accessed by all other parts in your code(and also by other javascripts).

while var x=5 will be familiar only to the block which it was declared on, (and it's descendants)

Note that although x=5 will be global, it will become global only when this line is execute! So if you place this inside a function, it will become global only after that function is called (for the first time).

It makes no difference - with the second one (x=5;) the variable is automatically declared if it does not yet exist.

I always use the first option however.