How JavaScript Works
Understanding how JavaScript runs behind the scenes will help you write better code and debug more efficiently. Let's break it down!
JavaScript is Single-Threaded
JavaScript uses a single-threaded model, meaning it can do one thing at a time. It follows a synchronous execution model, but thanks to features like callbacks, promises, and async/await, it can handle asynchronous operations efficiently.
The JavaScript Runtime Environment
JavaScript doesn’t work alone — it runs inside an environment (usually the browser or Node.js). Here’s what that environment includes:
1. JavaScript Engine
- Compiles and executes your code.
2. Web APIs
- These are features provided by the browser, like setTimeout, DOM, fetch, etc. JavaScript itself doesn’t know how to handle these, so it relies on the browser.
3. Callback Queue
- Stores async callbacks (like setTimeout, event listeners) that wait to be executed.
4. Event Loop
- Constantly checks if the call stack is empty. If it is, it pushes a task from the callback queue to the stack.
Execution Flow
Here's a simplified flow of how JS runs code:
- Code goes to the Call Stack
- JavaScript runs functions one by one, from top to bottom.
- If it hits a Web API (e.g., setTimeout), it sends the task to the Web API and continues.
- Once the Web API finishes, the callback is sent to the Callback Queue.
- The Event Loop checks if the call stack is empty. If so, it pushes the callback to the call stack and runs it.
Example
console.log("Start");
setTimeout(() => {
console.log("Inside setTimeout");
}, 0);
console.log("End");
Output
Start
End
Inside setTimeout
End
Inside setTimeout
Even though the setTimeout delay is 0, it runs last because it goes through the Web API and Callback Queue.