JavaScript InternalsRuntime Simulation

Snippet

The call stack, on its own — push on call, pop on return.

1function third() {

2 console.log("third");

5function second() {

6 console.log("second");

7 third();

10function first() {

11 console.log("first");

12 second();

13}

15first();

Three functions calling each other, top to bottom. There is no timer, no promise and no queue anywhere in this program.

The goal

Read the call stack fluently before anything asynchronous complicates it: a call pushes, a return pops, and only the top frame runs.

What to look for

Simulation notes

Clock readings are simulated, not measured: each step costs a fraction of a millisecond and timers fire a little late, as they do in a real runtime — but the jitter never changes the order anything runs in.

Press Play, or Step through one move at a time — this line explains each one.

Call stack1 frame

main()

Browser / host environmentclock 0ms

No timers running. The host provides far more than timers — fetch, DOM events, storage — but this simulation models only what its snippets use.

Event loop

blocked — stack busy

Task queue (macrotasks)0 waiting

No tasks queued.

Microtask queue0 waiting

No microtasks queued.

Console0 lines

Nothing logged yet.

Events0 / 10

Speed

Nothing has happened yet. Press Play or Step, then click any event here to jump back to that moment.

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