How JavaScript Handles Concurrency Behind the Scenes

Although JavaScript is single-threaded, meaning it can execute only one operation at a time, it uses an event-driven, non-blocking model to handle asynchronous tasks. The browser or Node.js runtime offloads time-consuming operations like network requests, timers, or animations to background APIs or threads. After these tasks finish, their callbacks are queued up, waiting for the main thread to be free before they can run. The JavaScript event loop manages this flow, allowing asynchronous operations to run smoothly without interrupting the main thread or causing the user interface to become unresponsive. If you’re working on complex asynchronous logic or API integrations, it can be highly beneficial to hire a JavaScript API developer to ensure robust and efficient implementation.

The short answer: Concurrency in JavaScript is powered by the event loop, callback queue, Web APIs, and microtasks, not by threads in the traditional sense. 

Wait, Isn’t JavaScript Single-Threaded? 

JavaScript operates on a single-threaded model, both in web browsers and in Node.js environments. This means it can handle only one task at a time during execution.. 

But thanks to its asynchronous runtime, JS can still handle many tasks seemingly in parallel, like: 

• Fetching data from APIs 

• Handling user events 

• Running timers 

• Reading files (in Node.js) 

The Building Blocks of Concurrency

1. The Call Stack

This is the part of JavaScript’s execution model where functions are added and removed as the code runs, known as the call stack. JavaScript processes synchronous code using a last-in, first-out (LIFO) approach, where the most recent function call is completed before earlier ones. 

function foo() { 
  console.log(“Hi”); 
} 
foo(); 

Output: 
Hi, Easy enough — but what happens with setTimeout or fetch()? 

2. Web APIs (or Node APIs)

When you use something like setTimeout(), the JS engine doesn’t wait for it to finish. Instead, the browser or Node environment offloads it to the Web API layer. 

setTimeout(() => console.log(“Later”), 1000); 

console.log(“Now”); 

Output: 

Now 

Later 

The setTimeout runs outside the main thread, and when it finishes, the callback is queued for execution.

3. Callback Queue (a.k.a. Task Queue)

When asynchronous tasks, such as timers or user events, are completed, their callbacks are placed in the callback queue. These callbacks remain in the queue and are only executed once the call stack has finished processing all current tasks. 

4. The Event Loop (The Scheduler)

The event loop is a continuously running process that manages the interaction between the call stack and the callback queue, ensuring JavaScript code runs in the proper sequence. When there are no more tasks in the call stack, the event loop retrieves the next function from the callback queue and pushes it onto the stack so it can be executed. 

• Is the call stack empty? 

• Once the call stack is empty, the event loop selects the next task from the callback queue and pushes it onto the stack for execution. 

This creates the illusion of multitasking, but it involves interleaving asynchronous tasks with synchronous code.

5. Microtasks (Promises, MutationObserver, etc.)

Microtasks are a special queue for high-priority async tasks (like .then() callbacks from Promises). 

Promise.resolve().then(() => console.log(“Microtask”)); 

setTimeout(() => console.log(“Macrotask”), 0); 

console.log(“Sync”); 

Output: 

Sync 

Microtask 

Macrotask 

Once the current script has completed, microtasks are handled immediately, before moving on to any callbacks in the task queue.

Real-World Example 

console.log(“Start”); 
setTimeout(() => { 
  console.log(“setTimeout”); 
}, 0); 
Promise.resolve().then(() => { 
  console.log(“Promise”); 
}); 
console.log(“End”); 

Output: 

Start 

End 

Promise 

setTimeout

What About Web Workers or Node.js Worker Threads? 

JS can use Web Workers (browser) or Worker Threads (Node.js) to run truly parallel code in separate threads. 

But they don’t share memory with the main thread; they communicate via messages. 

Summary 

• JavaScript is single-threaded, but supports concurrency via asynchronous APIs. 

• The event loop is the brain coordinating between sync code and async callbacks. 

• Promises and microtasks are faster than timers and user events. 

JS concurrency is about interleaving, not parallelism — unless you explicitly use threads or workers.