Why reassigning an object defined in the outer function to a same variable name as declared outside is giving error?

Question

Why typeof of b is undefined in the below code?

var b = function() {}

var a = function() {
var b = b
console.log('typeof function_b:', typeof b) 
}
a()

Answer 1

Because you're initialising a new variable inside the a function scope with the declaration var b.

var b gets initialised and ran before the value gets assigned (b = b), so it is assigning the just-initialised empty value to itself.

To alter the output, you can just skip the var declaration and typeof b outputs "function":

var b = function() {}

var a = function() {
    b = b;
    console.log('typeof function_b:', typeof b); // Outputs "function"
}
a();

Answer 2

That's because the variable first get declared (var b), and then assigned b = b. After declaring it, it is undefined in that scope, so you're assigning undefined to it.