WASM Architecture Decision

Decision framework for WebAssembly usage in browser extensions, including browser-specific loading patterns, architectural patterns, and performance considerations.

Decision Framework

When to Use WASM

Use Case	Recommendation	Rationale
Cryptographic operations	Strong yes	10-100x faster, constant-time
Image/video processing	Strong yes	Parallelizable, memory efficient
Compression/decompression	Yes	CPU-intensive, existing Rust libs
Complex parsing (binary)	Yes	Type safety, predictable perf
Scientific computing	Yes	Numerical precision, speed
Simple data transforms	No	Overhead exceeds benefit
DOM manipulation	No	JS required, no benefit
API calls/networking	No	I/O bound, not CPU bound
String manipulation	Maybe	Only for large-scale ops

Decision Matrix

Score each factor 1-5, then calculate totals:

Factor	Weight	Score
CPU intensity	3x	[1-5]
Data volume	2x	[1-5]
Performance criticality	2x	[1-5]
Existing Rust/C++ code	2x	[1-5]
Team Rust expertise	1x	[1-5]

Scoring guide:

• Total 30+: Strong WASM candidate
• Total 20-29: Consider WASM
• Total <20: Use JavaScript

Anti-Patterns

Pattern	Problem	Alternative
WASM for DOM access	Impossible, must call JS	Keep DOM in JS layer
WASM for simple logic	Overhead exceeds benefit	Native JS
Frequent WASM↔JS calls	Call overhead ~10μs each	Batch operations
Large data copies	Memory duplication	Use SharedArrayBuffer
Sync WASM in main thread	Blocks UI	Web Worker or async

Browser-Specific Loading Patterns

Chrome (MV3 Service Worker)

// Service worker has no DOM, but full WASM support
let wasmModule: WebAssembly.Module | null = null;

// Pre-compile on install for fast instantiation
chrome.runtime.onInstalled.addListener(async () => {
  const response = await fetch(chrome.runtime.getURL('wasm/module.wasm'));
  wasmModule = await WebAssembly.compileStreaming(response);
});

// Instantiate per-use (service worker may have terminated)
async function getWasmInstance(): Promise<WebAssembly.Instance> {
  if (!wasmModule) {
    const response = await fetch(chrome.runtime.getURL('wasm/module.wasm'));
    wasmModule = await WebAssembly.compileStreaming(response);
  }
  return new WebAssembly.Instance(wasmModule);
}

Firefox (MV3 or MV2)

// Firefox supports both event pages and service workers
// Event page approach (MV2) - has DOM access
let wasmInstance: WebAssembly.Instance | null = null;

async function initWasm(): Promise<WebAssembly.Instance> {
  if (wasmInstance) return wasmInstance;

  const response = await fetch(browser.runtime.getURL('wasm/module.wasm'));

  // Firefox supports streaming compilation
  const { instance } = await WebAssembly.instantiateStreaming(response);
  wasmInstance = instance;

  return instance;
}

Safari

// Safari has strict WASM policies
// 1. No streaming from cross-origin (must use buffer)
// 2. WASM files must be in extension bundle
// 3. Content-Type must be application/wasm

async function initWasmSafari(): Promise<WebAssembly.Instance> {
  const response = await fetch(browser.runtime.getURL('wasm/module.wasm'));

  // Safari fallback: no streaming compilation
  const bytes = await response.arrayBuffer();
  const { instance } = await WebAssembly.instantiate(bytes);

  return instance;
}

// Cross-browser wrapper
async function initWasmCrossBrowser(): Promise<WebAssembly.Instance> {
  const response = await fetch(browser.runtime.getURL('wasm/module.wasm'));

  if (typeof WebAssembly.instantiateStreaming === 'function') {
    try {
      const { instance } = await WebAssembly.instantiateStreaming(response);
      return instance;
    } catch {
      // Fallback on streaming failure
    }
  }

  // Safari and fallback path
  const bytes = await response.arrayBuffer();
  const { instance } = await WebAssembly.instantiate(bytes);
  return instance;
}

Content Script Loading

// Content scripts run in isolated world
// WASM must be in web_accessible_resources

async function loadWasmInContentScript(): Promise<WebAssembly.Instance> {
  // Use chrome.runtime.getURL for proper extension URL
  const wasmUrl = chrome.runtime.getURL('wasm/module.wasm');

  const response = await fetch(wasmUrl, {
    credentials: 'omit', // Don't send cookies
    cache: 'force-cache' // Cache aggressively
  });

  if (!response.ok) {
    throw new Error(`Failed to load WASM: ${response.status}`);
  }

  return initWasmCrossBrowser();
}

Architecture Patterns

Pattern 1: Background-Only WASM

WASM runs only in service worker, content scripts communicate via messaging.

┌─────────────────┐     message      ┌──────────────────┐
│ Content Script  │◄───────────────►│ Service Worker   │
│ (no WASM)       │                  │ (WASM loaded)    │
└─────────────────┘                  └──────────────────┘

Pros: Single WASM instance, simpler memory management
Cons: Message serialization overhead, latency
Best for: Infrequent operations, small data

// content-script.ts
async function processData(data: Uint8Array): Promise<Uint8Array> {
  const response = await chrome.runtime.sendMessage({
    type: 'WASM_PROCESS',
    data: Array.from(data) // Must serialize
  });
  return new Uint8Array(response.result);
}

// background.ts
chrome.runtime.onMessage.addListener((msg, sender, sendResponse) => {
  if (msg.type === 'WASM_PROCESS') {
    processWithWasm(new Uint8Array(msg.data))
      .then(result => sendResponse({ result: Array.from(result) }));
    return true;
  }
});

Pattern 2: Content Script WASM

WASM runs in each content script instance.

┌────────────────────────────────────────────────────────┐
│                     Tab 1                               │
│  ┌─────────────────┐     ┌──────────────────────────┐ │
│  │ Content Script  │────►│ WASM (per-tab instance)  │ │
│  └─────────────────┘     └──────────────────────────┘ │
└────────────────────────────────────────────────────────┘

Pros: No message latency, parallel processing
Cons: Memory per tab, startup per tab
Best for: Per-page processing, large data

// content-script.ts
import init, { process } from './wasm/module';

let wasmReady = false;

async function ensureWasm(): Promise<void> {
  if (wasmReady) return;
  await init(chrome.runtime.getURL('wasm/module_bg.wasm'));
  wasmReady = true;
}

async function processLocally(data: Uint8Array): Promise<Uint8Array> {
  await ensureWasm();
  return process(data);
}

Pattern 3: Web Worker WASM

WASM runs in dedicated worker, avoiding main thread blocking.

┌─────────────────┐     postMessage    ┌─────────────────┐
│ Content Script  │◄──────────────────►│ Web Worker      │
│ (main thread)   │                    │ (WASM loaded)   │
└─────────────────┘                    └─────────────────┘

Pros: Non-blocking, parallelizable
Cons: Setup complexity, message overhead
Best for: Heavy computation, real-time processing

// wasm-worker.ts
import init, { process } from './wasm/module';

self.onmessage = async (e) => {
  await init();

  const result = process(e.data.input);

  // Transfer buffer ownership (zero-copy)
  self.postMessage(
    { result: result.buffer },
    [result.buffer]
  );
};

// content-script.ts
const worker = new Worker(chrome.runtime.getURL('wasm-worker.js'));

function processAsync(data: Uint8Array): Promise<Uint8Array> {
  return new Promise((resolve) => {
    worker.onmessage = (e) => resolve(new Uint8Array(e.data.result));
    worker.postMessage(
      { input: data.buffer },
      [data.buffer] // Transfer ownership
    );
  });
}

Pattern 4: Offscreen Document (Chrome MV3)

Use offscreen document for DOM-dependent WASM operations.

// background.ts
async function ensureOffscreen(): Promise<void> {
  if (await chrome.offscreen.hasDocument()) return;

  await chrome.offscreen.createDocument({
    url: 'offscreen.html',
    reasons: ['DOM_PARSER', 'WORKERS'],
    justification: 'WASM processing with DOM'
  });
}

async function processViaOffscreen(data: Uint8Array): Promise<Uint8Array> {
  await ensureOffscreen();

  const response = await chrome.runtime.sendMessage({
    target: 'offscreen',
    type: 'WASM_PROCESS',
    data: Array.from(data)
  });

  return new Uint8Array(response.result);
}

Performance Benchmarks

Typical Performance Gains

Operation	JS (ms)	WASM (ms)	Speedup
SHA-256 (1MB)	45	8	5.6x
AES-256 encrypt	120	15	8x
JSON parse (10MB)	150	40	3.75x
GZIP compress	200	35	5.7x
Image resize	300	50	6x
Regex (complex)	80	25	3.2x
Simple sum	0.1	0.2	0.5x (slower!)

When JavaScript is Faster

Scenario	Why JS Wins
<1ms operations	WASM call overhead dominates
Single string ops	JS strings optimized
DOM manipulation	Must call JS anyway
Small arrays (<1KB)	Copy overhead dominates
JIT-optimized hot paths	V8/SpiderMonkey excellent

Benchmarking Template

async function benchmarkOperation(
  jsImpl: (data: Uint8Array) => Uint8Array,
  wasmImpl: (data: Uint8Array) => Uint8Array,
  data: Uint8Array,
  iterations: number = 100
): Promise<{ js: number; wasm: number; speedup: number }> {
  // Warm up
  jsImpl(data);
  wasmImpl(data);

  // Measure JS
  const jsStart = performance.now();
  for (let i = 0; i < iterations; i++) {
    jsImpl(data);
  }
  const jsTime = (performance.now() - jsStart) / iterations;

  // Measure WASM
  const wasmStart = performance.now();
  for (let i = 0; i < iterations; i++) {
    wasmImpl(data);
  }
  const wasmTime = (performance.now() - wasmStart) / iterations;

  return {
    js: jsTime,
    wasm: wasmTime,
    speedup: jsTime / wasmTime
  };
}

Memory Architecture

Extension Memory Limits

Context	Chrome	Firefox	Safari
Service Worker	128MB	512MB	128MB
Event Page	N/A	512MB	N/A
Content Script	Tab limit	Tab limit	Tab limit
Popup	128MB	128MB	128MB

Memory-Efficient Patterns

// Rust: Reuse buffers to avoid allocation
static mut BUFFER: Vec<u8> = Vec::new();

#[wasm_bindgen]
pub fn process_reuse(data: &[u8]) -> *const u8 {
    unsafe {
        BUFFER.clear();
        BUFFER.extend_from_slice(data);
        // Process BUFFER...
        BUFFER.as_ptr()
    }
}

// Return length separately
#[wasm_bindgen]
pub fn get_result_len() -> usize {
    unsafe { BUFFER.len() }
}

// TypeScript: Read result without copy
const ptr = wasmInstance.exports.process_reuse(inputPtr);
const len = wasmInstance.exports.get_result_len();
const memory = new Uint8Array(wasmInstance.exports.memory.buffer);
const result = memory.slice(ptr, ptr + len);

Streaming Processing

// Process large files in chunks to stay within memory limits
async function processLargeFile(
  file: File,
  chunkSize: number = 1024 * 1024 // 1MB chunks
): Promise<Uint8Array[]> {
  const results: Uint8Array[] = [];
  const reader = file.stream().getReader();

  while (true) {
    const { done, value } = await reader.read();
    if (done) break;

    const processed = await processChunkWithWasm(value);
    results.push(processed);
  }

  return results;
}

Related Resources

• wasm-extension-integration skill: Build pipeline and tooling
• wasm-integration-advisor agent: Suitability analysis
• wasm-decision-report style: Report format
• /add-wasm-module command: Scaffolding tool