Question

Are following snippets exactly equal? If no what is the deference?

var x = (function() {
    ... //a
    return function(){
        ... //b
    };
})();

vs.

var x;
{
    ... //a
    x = function(){
        ... //b
    };
}

Answer 1

There is a major difference: In JavaScript, blocks don’t induce a new variable scope. Therefore, you can’t define private variables in the // a code block. Compare

var x = (function() {
    var v = 42;
    return function(){
        return v;
    };
})();
// v; would yield ReferenceError: v is not defined, so you need to call x

and

var x;
{
    var v = 42;
    x = function(){
        return v;
    };
}
// v is 42 here, that's not what's intended.