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Call Stack: which function is running

Pushing and popping stack frames in the correct order.

When start() calls update(), execution of start() waits for the return. When update() calls damage(), the active function becomes damage(). On return, control moves back in the opposite order.

function damage() { return 10; }
function update() { return damage(); }
function start() { return update(); }
start();

This is the LIFO: last in, first out principle. The stack records where execution should continue after a nested call. The ECMA-262 specification defines a stack of execution contexts; V8’s actual machine-level frames may differ because of inlining and other optimizations.

In the simulation script represents the base of the running script in this diagram. The “Next action” button traces changes to the top frame. The diagram shows the logical order of calls, rather than a native stack dump.

If a synchronous call takes a long time to return, another JavaScript callback on the same thread cannot interrupt it on its own.

Call and return order

Let’s follow the changes to the top frame and the corresponding line of code.

1function damage() { return 10; }2function update() { return damage(); }3function start() { return update(); }4start();

CALL STACK · TOP AT THE TOP

scriptCurrently executing

OUTPUT—

The current script is ready to call start().

1function damage() { return 10; }2function update() { return damage(); }3function start() { return update(); }4start();

CALL STACK · TOP AT THE TOP

scriptWaiting for a return

start()Currently executing

OUTPUT—

Save the return location in script.

1function damage() { return 10; }2function update() { return damage(); }3function start() { return update(); }4start();

CALL STACK · TOP AT THE TOP

scriptWaiting for a return

start()Waiting for a return

update()Currently executing

OUTPUT—

start() waits for update() to return.

1function damage() { return 10; }2function update() { return damage(); }3function start() { return update(); }4start();

CALL STACK · TOP AT THE TOP

scriptWaiting for a return

start()Waiting for a return

update()Waiting for a return

damage()Currently executing

OUTPUT—

The top frame is damage(). Compute 10.

1function damage() { return 10; }2function update() { return damage(); }3function start() { return update(); }4start();

CALL STACK · TOP AT THE TOP

scriptWaiting for a return

start()Waiting for a return

update()Currently executing

OUTPUTreturn 10

damage() is popped; update() receives 10.

1function damage() { return 10; }2function update() { return damage(); }3function start() { return update(); }4start();

CALL STACK · TOP AT THE TOP

scriptWaiting for a return

start()Currently executing

OUTPUTreturn 10

update() is popped; start() receives 10.

1function damage() { return 10; }2function update() { return damage(); }3function start() { return update(); }4start();

CALL STACK · TOP AT THE TOP

scriptCurrently executing

OUTPUTreturn 10

The nested calls are complete; control has returned to the script.

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Key takeaway

A nested call runs before its calling function can continue.

Check the primary source ↗

ECMA-262 · Execution Contexts↗Execution contexts and their stack in the specification.