Why this output is 2 ?

Question

function calc(x) {
return x/2;
}
var z = calc(calc(8));
document.write(z);
//output is 2

I don't understand "calc(calc(8)); "
What does this first calc and second calc mean?
Can someone explain me ?
Thanks in advance.

Answer 1

calc(8) { return 8/2 } // 4
calc(4) { return 4/2 } // 2

Answer 2

I see! subsutituting! Now that make sence. Thank you!