Explanation Intermediate

Chapter 9 Updated April 21, 2026

Functions, Callbacks, Event Loop, JS Engine & setTimeout

First class functions, callbacks, the event loop, V8, and why setTimeout lies.

Full 12m
Revision 3m
Flow 2m

Functions are values in JS.

Functions can be assigned to variables, passed as arguments, returned, and stored in data structures.
Statement vs Expression: only difference is hoisting — statements are fully hoisted; expressions are undefined (var) or in TDZ (let/const).
Named Function Expression: the inner name is only accessible inside the function body.
Anonymous functions cannot stand alone — they are used as values only.

A function passed to another, called later.

Callbacks give JS access to the async world despite being synchronous and single-threaded.
setTimeout, event listeners, fetch, array methods — all built on callbacks.
Event listeners form closures over their outer variables — variables persist as long as the listener lives.
Heavy listeners on a page leak memory — always removeEventListener when done.

JS engine + Web APIs + queues + event loop.

JS Engine holds the Call Stack and Memory Heap. Web APIs (setTimeout, fetch, DOM) live in the browser, not the language.
Callback Queue holds setTimeout/setInterval/event-handler callbacks.
Microtask Queue holds promise callbacks (.then/.catch/.finally), queueMicrotask, MutationObserver — higher priority.
Event Loop is the gatekeeper: when the stack is empty, push from microtasks first, then macrotasks.

Sync → all microtasks → one macrotask → repeat.

All synchronous code runs first (including Promise constructor bodies).
The event loop then completely drains the microtask queue before touching macrotasks.
Only one macrotask is processed per cycle, then microtasks get another full pass.
Starvation: endless microtasks can indefinitely block the callback queue.

Parse → JIT compile → execute.

Parsing turns source into tokens and an Abstract Syntax Tree (AST).
JIT Compilation — Ignition interprets to bytecode; TurboFan optimizes hot paths to machine code.
Execution uses the Memory Heap (objects/variables) and Call Stack (execution contexts).
Orinoco is V8’s mark-and-sweep garbage collector.

It's a minimum delay, not an exact one.

setTimeout(cb, 5000) guarantees at least 5 seconds — if the stack is busy, the callback waits longer.
setTimeout(fn, 0) still goes through Web API → queue → event loop → stack. It only runs after all current sync code.
Use setTimeout(fn, 0) to defer less-critical work; never to “run immediately.”
The fix for late callbacks is never blocking the main thread in the first place.

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