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Atlas Whoff
Atlas Whoff

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WebAssembly in JavaScript: When and How to Use WASM

#javascript #performance #webdev #programming

WebAssembly in JavaScript: When and How to Use WASM

WebAssembly runs near-native performance code in the browser. Here's when it actually makes sense to use it.

What WASM Is Good For

WASM excels at:

  • CPU-intensive computation (video encoding, image processing, physics)
  • Porting existing C/C++/Rust code to the browser
  • Consistent performance across browsers

WASM is NOT needed for:

  • DOM manipulation (JS is faster for this)
  • Network requests
  • Most business logic
  • Anything that runs in <10ms anyway

Calling WASM From JavaScript 

// Load and instantiate a WASM module
const response = await fetch('module.wasm');
const buffer = await response.arrayBuffer();
const { instance } = await WebAssembly.instantiate(buffer);

// Call WASM functions directly
const result = instance.exports.fibonacci(40);
console.log(result); // Returns in ~1ms vs ~2000ms in pure JS
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Rust to WASM With wasm-pack 

// src/lib.rs
use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub fn process_image(data: &[u8], width: u32, height: u32) -> Vec<u8> {
    // CPU-intensive image processing
    data.chunks(4)
        .flat_map(|pixel| {
            let r = pixel[0];
            let g = pixel[1];
            let b = pixel[2];
            let gray = (0.299 * r as f32 + 0.587 * g as f32 + 0.114 * b as f32) as u8;
            vec![gray, gray, gray, pixel[3]]
        })
        .collect()
}
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wasm-pack build --target web
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import init, { process_image } from './pkg/my_module.js';

await init();

const canvas = document.querySelector('canvas');
const ctx = canvas.getContext('2d');
const imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
const processed = process_image(imageData.data, canvas.width, canvas.height);
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Real-World WASM Use Cases

  • Figma: complex vector rendering engine in WASM
  • Photoshop for Web: entire image processing pipeline
  • SQLite in the browser: @sqlite.org/sqlite-wasm
  • ffmpeg.wasm: client-side video transcoding

Performance Comparison 

// Benchmark: fibonacci(40)
console.time('js');
fibJS(40); // ~2000ms
console.timeEnd('js');

console.time('wasm');
fibWasm(40); // ~1ms
console.timeEnd('wasm');
// 2000x speedup for this specific workload
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For most SaaS applications, Web Workers (pure JS, multithreaded) are sufficient before reaching for WASM. The Ship Fast Skill Pack includes patterns for both Web Workers and WASM integration.

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