javascript variable scope in function confusion

Question

Here are 2 javascript functions

var a = 10;
function abcd()
{
 alert(a);//alerts 10
 a=5;
}

And another code is this

var a = 10;
function abcd()
{
 alert(a);//alerts undefined
 var a=5;
}

In both function assignment/declaration is after alert() call. Then why alert message are 10 and undefined respectively?

Answer 1

That's because your variable gets "hoisted" up of its containing scope by the interpreter when you declare it. So your code ends up being interpreted like this:

function abcd()
{
 var a;
 alert(a); //alerts undefined
 a = 5;
}

To avoid this kind of confusion, you can follow some practices that will keep things in place, like declaring your local-scoped (that is, variables declared with the keyword var inside a function scope) variables right in the beginning of the function.

Note that, as you can read from the article, this happens with nested functions aswell.