JavaScript Interview Question #154

Why does setTimeout(fn, 0) not execute immediately?

Performance, Browser APIs, Rendering & Modern Web Architecture Senior Advanced

Quick Interview Answer

A zero-millisecond timeout means the callback becomes eligible to run after the minimum scheduling delay; it does not interrupt currently executing JavaScript. The callback is placed into the runtime's task scheduling mechanism and must wait for the current call stack and relevant microtasks to finish.

Detailed Explanation

This is why synchronous statements and already-scheduled Promise microtasks can execute before a zero-delay timer.

Code Example

console.log("A");

setTimeout(() => {
    console.log("Timer");
}, 0);

Promise.resolve().then(() => {
    console.log("Promise");
});

console.log("B");

// A, B, Promise, Timer