Difference between variable declaration syntaxes in Javascript (including global variables)?

Question

Is there any difference between declaring a variable:

var a=0; //1

...this way:

a=0; //2

...or:

Answer 1

In global scope there is no semantic difference.

But you really should avoid a=0 since your setting a value to an undeclared variable.

Also use closures to avoid editing global scope at all

(function() {
   // do stuff locally

   // Hoist something to global scope
   window.someGlobal = someLocal
}());

Always use closures and always hoist to global scope when its absolutely neccesary. You should be using asynchronous event handling for most of your communication anyway.

As @AvianMoncellor mentioned there is an IE bug with var a = foo only declaring a global for file scope. This is an issue with IE's notorious broken interpreter. This bug does sound familiar so it's probably true.

So stick to window.globalName = someLocalpointer