Is it possible to overwrite the undefined in javascript?

Question

like

function myFunction(){
    var undefined = "abc";
}

If its possible then how to restrict not to allow that?

Answer 1

Is it possible to overwrite the undefined in javascript?

If by "the undefined" you mean the global undefined variable, then no. Since EcmaScript 5, it is specified as non-writable. However, older browsers don't adhere that spec, so it is overwritable in legacy engines. You cannot really prevent it in them, but always reset it by undefined = void 0;. If you still worry and want to know how to protect your own scripts, check the question How dangerous is it in JavaScript, really, to assume undefined is not overwritten?.

like function myFunction(){ var undefined = "abc"; }

That's a different thing. You can always declare a local variable with the name undefined (shadowing the global one) and assign arbitrary values to it.

Answer 2

Some browsers allow it, the best way to restrict it is avoiding it.

But... some are using this technique to preserve the undefined:

(function(undefined){

})()

They get a variable called undefined but don't pass a value which gives undefined the undefined value.

From jQuery's source code:

(function( window, undefined ) {
    ...
    ...
})(window);