Avoid This Danger: 3 Reasons Not to Build a Synchronous JavaScript API Wrapper

Let’s be honest. Sometimes, when you’re dealing with an API that has sequential steps, you might look at the neat, top-to-bottom flow of synchronous code and wish you could just make your API calls work like that. No .then() chains, no await keywords – just call a function and poof, you have the result on the next line. The idea of a synchronous API wrapper JavaScript might cross your mind.

It sounds simple, right? Get rid of all that asynchronous complexity and write code that looks beautifully linear. But hold on a second. While the idea is tempting, actually building a truly synchronous API wrapper in standard JavaScript environments (like browsers or Node.js) is almost always a terrible idea and goes against the fundamental design of how JavaScript handles operations that take time.

In this post, we’ll explore why you might want a synchronous wrapper, why JavaScript is built to be asynchronous, and give you 3 powerful reasons why you should avoid building a blocking synchronous API wrapper, focusing instead on better, non-blocking patterns.

Why Would Anyone Want a Synchronous API Wrapper?

The appeal is understandable. When you write synchronous code, execution flows sequentially from one line to the next.

const data = readFileSync('config.json'); // Execution pauses here until read is done
const parsedConfig = JSON.parse(data);
console.log(parsedConfig); // This line runs ONLY after reading and parsing are complete

This linear flow is easy to read and reason about. When dealing with a series of API calls where each step depends on the result of the previous one, mapping this to a synchronous model feels intuitive. You call getUser(), then getOrders(user.id), then getDetails(order.id) – it mirrors the logical steps perfectly without needing explicit asynchronous handling at each point in the calling code.

The Big Problem: JavaScript is Asynchronous by Design for I/O

Here’s the catch. JavaScript’s design, especially in environments like browsers and Node.js, relies heavily on an event loop. This allows it to perform non-blocking operations. When you initiate something time-consuming like a network request (fetching data from an API) or reading a file, JavaScript doesn’t just stop and wait. Instead, it tells the environment (browser or Node) to perform the task and provides a callback function (or a Promise) to be executed later when the task is finished. Meanwhile, the JavaScript engine is free to run other code, keep the user interface responsive, or handle other requests.

API calls are fundamentally Input/Output (I/O) operations. They take an unpredictable amount of time due to network latency, server processing, etc.

Why Blocking the Main Thread is Bad (The 3 Reasons You Should Avoid Sync Wrappers)

Trying to force these inherently asynchronous I/O operations into a synchronous, blocking model leads to severe problems. Here are the main reasons why you should avoid building a synchronous API wrapper:

Reason 1: It Freezes Your Application

• In the Browser: If you make a synchronous API call on the main browser thread, the entire browser tab will freeze. The user won’t be able to click buttons, scroll, type, or interact with the page at all until the API call completes (which could be milliseconds or several seconds!). This provides a terrible user experience. Modern browsers even show warnings if you try this extensively.
• In Node.js: If you make a synchronous API call on the main Node.js thread (the event loop), your server will stop processing any other requests until that call finishes. This means if multiple users hit your server simultaneously, they will all be stuck waiting in line behind any blocking API calls. This is highly inefficient and makes your application unscalable and unresponsive.

Reason 2: It Makes Error Handling & Timeouts Difficult

In a blocking synchronous model, handling potential network errors, server errors, or timeouts becomes awkward and often less robust compared to asynchronous patterns like Promises or Async/Await. These modern patterns have built-in mechanisms (.catch(), try...catch) designed specifically for managing the uncertain outcomes and potential failures of asynchronous operations gracefully. Forcing a synchronous wrapper often means complex, less standardized error handling logic.

Reason 3: It Breaks Standard JavaScript Practices and Libraries

Almost all modern JavaScript libraries and built-in APIs that perform I/O are designed to be asynchronous. They return Promises or expect callbacks. By building a synchronous wrapper, you’re fighting against the ecosystem. You won’t be able to easily integrate with other asynchronous code or take advantage of standard tools like Promise.all or the clean syntax of async/await within your “synchronous” wrapper structure itself.

Can We Really Build a Truly Synchronous Wrapper? (Technically, But…)

While strongly discouraged, are there any ways to achieve something that looks synchronous?

1. XMLHttpRequest Sync (Deprecated in Browsers): The old XMLHttpRequest API did have a synchronous flag. xhr.open(method, url, false);. Do not use this in new code, especially not in browsers. It’s deprecated for the very reasons mentioned above (freezing). It might technically work in very limited server-side scripting scenarios, but even there, it’s bad practice.
2. Web Workers (Not Main Thread Sync): In the browser, you could potentially move the blocking API call into a Web Worker. The code inside the worker runs synchronously relative to itself, but communication between the main thread and the worker is asynchronous (using postMessage). So, while it prevents the main thread from freezing, the overall interaction with your “wrapper” from the main thread is still asynchronous.
3. Top-Level Await (Limited Scope): In modern JavaScript modules (ES Modules) in certain environments (like Node.js or recent browser support), you can use await at the top level of a module. This makes the module loading process wait for an asynchronous operation to complete.

   // apiWrapper.js (This file is an ES Module)
   import fetch from 'node-fetch'; // Or standard fetch in browser
   
   const config = await fetch('/config').then(res => res.json());
   
   export const API_KEY = config.apiKey; // This line waits for fetch to complete
   

   When you import { API_KEY } from './apiWrapper.js', the import process pauses until the fetch is done. However, this only makes the initial loading synchronous; subsequent calls within your application still need to handle asynchronicity unless they also use top-level await (which is only for the module’s initial setup). This isn’t a general solution for making arbitrary API calls synchronous within your running application logic.

None of these methods provide the simple, main-thread-blocking synchronous API call abstraction you initially desired without significant drawbacks or limitations.

The Better Approach: Writing Asynchronous Code That Feels Synchronous

Instead of fighting the nature of JavaScript, embrace its asynchronous patterns. The good news is that modern JavaScript with Promises and Async/Await makes writing asynchronous code that looks and feels very clean and almost synchronous for sequential operations.

async function fetchDataAndProcess() {
  try {
    const user = await fetch('/users/123').then(res => res.json());
    console.log('Got user:', user);

    const orders = await fetch(`/orders/${user.id}`).then(res => res.json());
    console.log('Got orders:', orders);

    // This looks sequential, just like synchronous code!

  } catch (error) {
    console.error('Failed to fetch data:', error);
  }
}

fetchDataAndProcess();
console.log('This line runs immediately, the app is NOT blocked!');

Using async/await allows you to write asynchronous sequences in a linear style, avoiding nested callbacks while keeping the application responsive because the await keyword non-blockingly pauses the function’s execution, not the entire JavaScript engine.

When building an API wrapper, design its methods to return Promises:

class ApiWrapper {
  async getUser(userId) {
    const response = await fetch(`/users/${userId}`);
    if (!response.ok) throw new Error(`HTTP error! status: ${response.status}`);
    return response.json(); // Returns a Promise
  }

  async getOrders(userId) {
     const response = await fetch(`/orders/${userId}`);
     if (!response.ok) throw new Error(`HTTP error! status: ${response.status}`);
     return response.json(); // Returns a Promise
  }
  // ... other methods returning Promises
}

const api = new ApiWrapper();

// Now you use the async wrapper methods with await
async function processUserData(id) {
  try {
    const user = await api.getUser(id);
    const orders = await api.getOrders(user.id);
    console.log('Processed:', { user, orders });
  } catch (err) {
    console.error('Processing failed:', err);
  }
}

processUserData(123);

This async wrapper is easy to use, leverages standard JavaScript patterns, and most importantly, keeps your application responsive.

Recap: Synchronous vs. Asynchronous Wrappers

Conclusion

The desire for simple, linear code is valid, but trying to achieve it by building a blocking synchronous API wrapper in JavaScript environments designed for asynchronicity is detrimental. It leads to frozen UIs, unresponsive servers, and code that is difficult to maintain and integrate.

Instead of attempting to build a synchronous API wrapper JavaScript developers should focus on mastering Promises and Async/Await. These patterns allow you to write asynchronous code that looks clean and sequential using async/await, providing the readability you desire without blocking the critical main thread. Design your API wrappers to return Promises, and consume them using async/await for the best of both worlds.

Embrace the asynchronous nature of JavaScript – your users and your server will thank you!

Have you ever struggled with asynchronous code or been tempted by a synchronous approach? Share your experiences and challenges in the comments below!