Why the answer of counter = 0?

var counter = 0;
function add(){
    var counter = 0;
    counter+=1;
}
add();
add();
add();
console.log(counter); //output: 0

also how the GEC (Global execution context) and FEC (Functional execution context) will be created?

>Solution :

Lets take a closer look:

1. The add function has an internally defined counter variable. So it does not change the globally defined counter variable.

2. At the first line you have var counter = 0;. According to (1) there is no code to change the value of the globally defined counter. So at the last line the value would be 0.