What is the event loop in JavaScript and how does it work?
Aug 04, 2025 pm 01:54 PM
JavaScript's event loop implements non-blocking asynchronous execution by managing call stacks and task queues. 1. When the code is executed, the functions are put into the stack in turn; 2. Asynchronous operations (such as setTimeout and fetch) are handed over to the browser API for processing, and after completion, their callbacks enter the task queue; 3. Continuous checking of the event loop: When the call stack is empty, priority is given to clear the micro-task queue (such as Promise callback), and then execute a macro task (such as setTimeout); 4. Therefore, even if setTimeout is 0 milliseconds, it will be executed after synchronizing the code and the micro-task to ensure that the current task is not interrupted; 5. This mechanism enables JavaScript to respond efficiently to asynchronous operations although it is single-threaded.
The event loop is a core part of JavaScript's concurrency model that enables non-blocking execution, allowing JavaScript to handle asynchronous operations like times, HTTP requests, and user interactions—event being single-threaded.
How JavaScript Handles Code Execution
JavaScript runs on a single call stack, meaning it can only execute one piece of code at a time. When you call a function, it gets pushed onto the stack; when it returns, it's popped off. But if a task takes a long time (like waiting for a server response), blocking the stack would freeze the entire page.
To avoid this, JavaScript offloads slow tasks to browser APIs (like fetch , setTimeout , or DOM events). These run in the background, and once they complete, their callbacks are placed in a task queue (or callback queue ).
The Role of the Event Loop
The event loop continuously checks two things:
- Is the call stack empty?
- Are there any callbacks in the task queue?
It only pushes a callback from the queue onto the stack when the stack is completely empty. This ensures that currently running code finishes before any asynchronous callback runs.
Here's a simplified breakdown:
- Call Stack : Where functions are executed.
- Web APIs : Browser-provided APIs that handle async operations (eg,
setTimeout,fetch). - Callback Queue (Task Queue) : Holds callbacks ready to be executed.
- Event Loop : Monitors the call stack and queue, moving callbacks to the stack when possible.
Example to Illustrate
console.log("1");
setTimeout(() => {
console.log("2");
}, 0);
console.log("3"); You might expect this to log 1 , 2 , 3 —but it logs 1 , 3 , 2 .
Why?
-
"1"is logged immediately. -
setTimeoutis sent to a Web API, which waits 0ms, then puts its callback in the task queue. -
"3"is logged next because it's part of the main script (synchronous code). - Only after the call stack is clear does the event loop pick up the
setTimeoutcallback and log"2".
Microtasks vs Macrotasks
Not all async callbacks are treated equally. There are two types of queues:
- Macrotask Queue :
setTimeout,setInterval, I/O, UI rendering. - Microtask Queue :
Promise.then/catch/finally,queueMicrotask.
The event loop always processes the microtask queue entirely after each macrotask, before going back to the macrotask queue.
Example:
console.log("Start");
Promise.resolve().then(() => console.log("Microtask"));
setTimeout(() => console.log("Timeout"), 0);
console.log("End");Output:
Start End Microtask Timeout
Even though both the promise and timeout are ready quickly, the microtask runs before the next macrotask.
Key Takeaways
- The event loop enables async behavior in JavaScript by managing the call stack and task queues.
- It ensures the main thread isn't blocked by delegating long-running tasks to browser APIs.
- Microtasks (like Promises) have higher priority than macrotasks (like
setTimeout). - The loop checks: stack empty? → run microtasks → run one macrotask → repeat.
Basically, it's what makes JavaScript feel fast and responsive, even with heavy async operations.
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