How to Use WebAssembly to Speed Up CPU-Intensive Tasks in the Browser

JavaScript is powerful, but when it comes to CPU-intensive tasks like image processing, cryptographic operations, or simulation logic, performance can become an issue. This is where WebAssembly (Wasm) shines. In this guide, you'll learn how to compile a C function to WebAssembly and use it in a JavaScript-based web app to dramatically improve performance. What Is WebAssembly? WebAssembly is a low-level binary instruction format that runs in the browser at near-native speed. It allows you to compile code from languages like C, C++, and Rust and use it directly in your frontend code. Use Case: Fast Prime Number Checker Let’s say we want to determine if a number is prime—a task that can get costly with large numbers. We'll write a simple C function and compile it to WebAssembly. Step 1: Install Emscripten Follow the official installation guide to get Emscripten set up: git clone https://github.com/emscripten-core/emsdk.git cd emsdk ./emsdk install latest ./emsdk activate latest source ./emsdk_env.sh Step 2: Write the C Code Save this as isprime.c: #include bool is_prime(int num) { if (num

Apr 19, 2025 - 06:39

How to Use WebAssembly to Speed Up CPU-Intensive Tasks in the Browser

JavaScript is powerful, but when it comes to CPU-intensive tasks like image processing, cryptographic operations, or simulation logic, performance can become an issue. This is where WebAssembly (Wasm) shines. In this guide, you'll learn how to compile a C function to WebAssembly and use it in a JavaScript-based web app to dramatically improve performance.

What Is WebAssembly?

WebAssembly is a low-level binary instruction format that runs in the browser at near-native speed. It allows you to compile code from languages like C, C++, and Rust and use it directly in your frontend code.

Use Case: Fast Prime Number Checker

Let’s say we want to determine if a number is prime—a task that can get costly with large numbers. We'll write a simple C function and compile it to WebAssembly.

Step 1: Install Emscripten

Follow the official installation guide to get Emscripten set up:

git clone https://github.com/emscripten-core/emsdk.git
cd emsdk
./emsdk install latest
./emsdk activate latest
source ./emsdk_env.sh

Step 2: Write the C Code

Save this as isprime.c:

#include 

bool is_prime(int num) {
  if (num <= 1) return false;
  for (int i = 2; i * i <= num; i++) {
    if (num % i == 0) return false;
  }
  return true;
}

Step 3: Compile to WebAssembly

emcc isprime.c -s WASM=1 -s EXPORTED_FUNCTIONS='["_is_prime"]' -o isprime.js

This will output isprime.js and isprime.wasm.

Step 4: Use It in JavaScript

Now load the WebAssembly module in your frontend code:

Performance Comparison

Run the same function in JavaScript and WebAssembly on large inputs and compare execution times using console.time. You’ll notice significant speed improvements especially for higher ranges.

When to Use WebAssembly

Heavy mathematical computations
Real-time audio/video processing
Games and physics engines
Image manipulation

Conclusion

WebAssembly allows developers to offload compute-heavy tasks from JavaScript, providing a massive performance boost where needed. It’s especially useful when you're pushing the limits of what's possible in the browser.

If this guide helped, you can support my work at: buymeacoffee.com/hexshift