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JavaScript's event loop manages asynchronous operations by coordinating call stacks, web APIs, and task queues. 1. The call stack executes synchronous code, and when encountering asynchronous tasks, it is handed over to the Web API for processing; 2. After completing the task in the background, the Web API places the callbacks into the corresponding queue (macro task or micro task); 3. The event loop checks whether the call stack is empty. If it is empty, the callback is taken out from the queue and pushed into the call stack for execution; 4. Micro tasks (such as Promise.then) take precedence over macro tasks (such as setTimeout); 5. Understanding the event loop helps to avoid blocking the main thread and optimize the code execution order.
You've probably heard that JavaScript is single-threaded and uses an event loop to handle asynchronous operations. But how exactly does it all work together? Let's break down the event loop in a way that makes sense for real-world development.
The Call Stack: Where Code Gets Executed
Think of the call stack like a to-do list for your JavaScript code. When a function is called, it gets added to the top of the stack. Once it finishes running, it pops off.
For example:
function saysHello() {
console.log("Hello");
}
sayHello(); Here, sayHello gets pushed onto the stack, runs, then pops off. Simple enough. But what happens when you throw in something async, like a setTimeout ?
The call stack doesn't hang around waiting for async stuff — it just keeps going. That's where the rest of the event loop comes in.
Web APIs and the Callback Queue
When you use something like setTimeout , fetch , or DOM events, they're handed off to Web APIs (part of the browser), not handled directly by JS. These APIs do their thing in the background.
Once they're done — say, after 100ms has passed for a setTimeout — they push the callback into the callback queue .
But again, that callback doesn't run right away. It just sits there until the call stack is totally empty.
The Event Loop: Making It All Flow
This is the glue that connects everything. The event loop's job is simple: check if the call stack is empty. If it is, look in the callback queue and push the next item onto the stack.
That's why even if you set a timeout to 0ms, it still waits until the current code finishes. Try this:
console.log("Start");
setTimeout(() => {
console.log("Timeout");
}, 0);
Promise.resolve().then(() => {
console.log("Promise");
});
console.log("End");You'll see:
Start End Promise Timeout
Why the promise first? Because microtask queue (for promises) has higher priority than the regular callback queue.
Microtasks vs Regular Tasks
- Microtasks include things like
Promise.then,MutationObserver, andqueueMicrotask. - Macrotasks are things like
setTimeout,setInterval, and I/O events.
The event loop always clears the microtask queue before picking up the next macrotask. This can lead to some surprise behavior if you're not expecting it — especially with long chains of promises.
A few quick points to remember:
- Microtasks run before the next render or paint.
- Too many microtasks can block rendering or user interaction.
- Avoid doing heavy work inside
.then()if possible.
Practical Takeaways for Everyday Coding
So how does this help you write better code?
- Don't assume
setTimeout(fn, 0)will run immediately. It waits for the stack and other microtasks. - Use
Promise.thenfor async control flow, but be aware of execution order. - If you want to defer something until after rendering,
setTimeout(fn, 0)is actually more reliable than a promise. - Avoid blocking the main thread with large computings — consider Web Workers.
And one more thing: tools like DevTools won't show you the event loop directly, but understanding it helps make sense of why your app behaves the way it does under load or during complex async flows.
Basically, the event loop isn't magic — it's just a smart coordination of the stack, browser APIs, and queues. Once you get comfortable with how each part plays its role, debugging async issues become a lot easier.
The above is the detailed content of JS roundup: a deep dive into the JavaScript event loop. For more information, please follow other related articles on the PHP Chinese website!
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