Question

var x = 3;
(function (){
  console.log('before', x);
  var x = 7;
  console.log('after', x);
  return ;
})();

In the above code var X is initialized globally. So inside the function the first console.log should print “before 3” but i don’t get it. The reason is that i am trying to re-declare the global variable.

Can somebody explain why this is happening?

Answer 1

The JavaScript parser does Variable Hoisting when parsing your code. This means that any variable declaration will be moved to the top of the current scope, thus in your case, this code will get executed:

var x = 3;
(function (){
  var x;
  console.log('before', x);
  x = 7;
  console.log('after', x);
  return ;
})();

So your local variable x gets declared at first with an initial value of undefined.

This should explain, why you get an “beforeundefined” for the first console.log().