What is the event loop in JavaScript?

JavaScript runs one thing at a time, on the call stack. Timers and network requests are done by the browser in the background. When they finish, their callbacks wait in a queue, and the event loop moves them to the call stack when the stack is empty. Promise callbacks (microtasks) go before timer callbacks (macrotasks).

console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => console.log("3"));
console.log("4");

Output:

The normal code runs first (1, 4), then the promise callback (3), then the timer callback (2), even with 0 milliseconds.

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