Question

I have two functions, one function calling the other.

Below is a simple analogy I created of the structure of code which I have:

function test1(){
    alert(a);//a is not identified here even when test1() is called under test2 which has a defined in scope
    alert(b);//b is identified here
}

function test2(){
    var a       =       'a';//Defining a
    test1();//Calling test1()
}
var b   =   'b';
test2();

As per the code above, function test1() is able to identify variable b but not variable a.

My question is that why variable a is not in the scope of test1 function even when we are calling test1() inside test2() which defines variable a?

Thanks in advance.

Answer 1

Functions remember their scope chain at the time they’re being created, not at the time they’re called.

Note that b gets “hoisted”, so your code actually does this:

var b; // push b on scope chain

function test1(){ // store scope chain [b]
    alert(a);
    alert(b);
}

function test2(){ // store scope chain [b]
    var a; // push a on scope chain
    a = 'a';
    test1(); //Calling test1() which still has scope chain [b], not [b,a]
}
b   =   'b';
test2();