Expecting undefined in Javascript

Question

I was seeing some Javascript code and I stumbled upon something like this:

function() {
    if(true) {
        var a = 5;
    }
    alert(a);
}

I was pretty sure this would output undefined but it didn't ? Can someone tell me why?

Answer 1

JavaScript has function level scope, not block level scope.

The var statement is hoisted so your code is equivalent to:

function() {
    var a;
    if(true) {
        a = 5;
    }
    alert(a);
}

If JavaScript had block level scope, then it still wouldn't output undefined. Since a would be undeclared in the alert statement, you would trigger a reference error.

Answer 2

The reason this works is a result of what is called hoisting. Hoisting moves the declaration of the variable to the top of the scope. So your function really looks like this:

function() {
 var a;
 if(true) {
  a = 5;
 }
 alert(a);
}

"Because variable declarations (and declarations in general) are processed before any code is executed, declaring a variable anywhere in the code is equivalent to declaring it at the top. This also means that a variable can appear to be used before it's declared. This behaviour is called "hoisting", as it appears that the variable declaration is moved to the top of the function or global code." - var MDN