javascript - Why does calling setTimeout with parenthesis not start a new callstack?

Question

The following code has a new callstack when the debugger fires in d (jsfiddle here)

function c() {
    setTimeout( d, 1000 );
}

function d() {
    debugger;   
}

c();

If we modify the code to use setTimeout( d(), 1000 ); which has brackets (parenthesis:)

function c() {
    setTimeout( d(), 1000 );
}

function d() {
    debugger;   
}

c();

then the callstack has both c() and d() (jsfiddle here). Why?

Answer 1

You are not passing setTimeout the function d in the second example; you are instead passing d(), which is the result of calling d.

The result of calling d is undefined since it returns nothing, which converts to the string "undefined", which is then evaled, doing... precisely nothing.

---

With regard to callstacks, since you are calling d inside of c, that is why you see c in the callstack. To clarify, your second example is the same as

function c() {
    var temp = d();
    setTimeout(temp, 1000);
}

function d() {
    debugger;   
}

c();