JavaScript Runtimes Comparison

A comparison of the four major JavaScript runtime environments: Browser, Node.js, Deno, and Bun.

Overview

Architecture

Browser

• Sandboxed environment: Runs in an isolated context with strict security policies
• Event loop: Single-threaded with Web APIs for async operations (setTimeout, fetch, DOM events)
• Multi-engine landscape: V8 (Chrome/Edge), SpiderMonkey (Firefox), JavaScriptCore (Safari)
• Web APIs: DOM, Fetch, Web Workers, Service Workers, WebSocket, IndexedDB, Canvas, WebGL, WebGPU

// Browser-specific APIs
const response = await fetch('/api/data');
const data = await response.json();

// DOM manipulation
document.querySelector('#app').textContent = data.message;

// Web Worker for CPU-intensive tasks
const worker = new Worker('worker.js');
worker.postMessage({ task: 'compute', payload: largeData });
worker.onmessage = (e) => console.log(e.data.result);

// Service Worker for offline support
navigator.serviceWorker.register('/sw.js');

Node.js

• V8 engine: Same engine as Chrome, with libuv for async I/O
• Event loop: Single-threaded with worker_threads for CPU-intensive tasks
• CommonJS & ESM: Supports both module systems (CJS historically, ESM increasingly default)
• C++ addons: Native bindings via N-API for performance-critical code

import { readFile } from 'node:fs/promises';
import { createServer } from 'node:http';

// Built-in HTTP server
const server = createServer(async (req, res) => {
  const data = await readFile('./data.json', 'utf-8');
  res.writeHead(200, { 'Content-Type': 'application/json' });
  res.end(data);
});

server.listen(3000);

// Worker threads for CPU-intensive tasks
import { Worker } from 'node:worker_threads';
const worker = new Worker('./heavy-computation.js');

Deno

• V8 engine + Rust core: Runtime built in Rust using Tokio async runtime
• Secure by default: Requires explicit permissions for file, network, and environment access
• Web-standard APIs: Aligns with browser APIs where possible (Fetch, Web Streams, Web Crypto)
• Built-in toolchain: Formatter, linter, test runner, bundler, documentation generator, LSP

// TypeScript works out of the box - no config needed
import { serve } from "https://deno.land/std/http/server.ts";

// Uses Web standard Fetch API
const response = await fetch("https://api.example.com/data");
const data = await response.json();

// File system with explicit permissions
// Run with: deno run --allow-read --allow-net server.ts
const file = await Deno.readTextFile("./data.json");

// Built-in HTTP server (Web standard API)
Deno.serve({ port: 3000 }, async (req) => {
  return new Response(JSON.stringify({ hello: "world" }), {
    headers: { "Content-Type": "application/json" },
  });
});

Bun

• JavaScriptCore engine: Uses WebKit's engine (from Safari), optimized for startup and execution speed
• Written in Zig: Low-level language with manual memory management for maximum performance
• All-in-one toolkit: Runtime, bundler, test runner, package manager in a single binary
• Node.js compatible: Designed as a drop-in replacement for Node.js

// TypeScript works natively
import { serve } from "bun";

// Bun's optimized HTTP server
serve({
  port: 3000,
  fetch(req) {
    return new Response(JSON.stringify({ hello: "world" }), {
      headers: { "Content-Type": "application/json" },
    });
  },
});

// Fast file I/O with Bun APIs
const file = Bun.file("./data.json");
const data = await file.json();

// Built-in SQLite
import { Database } from "bun:sqlite";
const db = new Database("mydb.sqlite");

API Comparison

Web Standard APIs

Runtime-Specific APIs

Node.js Compatibility

Module System

Performance Benchmarks

HTTP Server Throughput (req/s)

Startup Time

Package Install Speed

TypeScript Execution

Built-in Tooling

Security Model

# Deno: explicit permissions
deno run --allow-read=./data --allow-net=api.example.com server.ts

# Deno: allow all (not recommended for production)
deno run -A server.ts

# Node.js: experimental permission model (v20+)
node --experimental-permission --allow-fs-read=./data server.js

Framework & Library Support

Web Frameworks

Meta-Frameworks (SSR / Fullstack)

ORM & Database

Deployment

Edge / Serverless Platforms

Containerization

Community & Ecosystem

Use Cases

Choose Browser When

• Building client-side web applications (the only option for end-user web)
• Leveraging DOM APIs and browser-specific features (Canvas, WebGL, WebGPU)
• Need sandboxed execution with strict security boundaries
• Building PWAs with Service Workers and offline support

Choose Node.js When

• Building production backend services with maximum ecosystem compatibility
• Need the largest package ecosystem and widest library support
• Working with enterprise infrastructure that expects Node.js
• Team has existing Node.js expertise and codebase
• Require mature tooling and battle-tested stability

Choose Deno When

• Security is a priority and want permissions by default
• Prefer Web standard APIs and modern JavaScript conventions
• Want an all-in-one toolchain (fmt, lint, test, bench, doc, lsp)
• Building for Deno Deploy edge platform
• Starting new projects without legacy CommonJS dependencies
• Working with TypeScript-first workflows without configuration

Choose Bun When

• Performance is critical (HTTP throughput, startup time)
• Want the fastest package manager and install times
• Need native TypeScript execution with minimal overhead
• Building performance-sensitive APIs and microservices
• Using Elysia or other Bun-optimized frameworks
• Want built-in SQLite for embedded database use cases

Summary

All three server-side runtimes are converging on Web standard APIs (Fetch, Web Streams, Web Crypto) and improving Node.js compatibility. Node.js remains the safe choice for production with the largest ecosystem. Deno leads in security, tooling, and standards compliance. Bun pushes the performance envelope and offers the best developer experience for speed-sensitive workflows. The Browser remains the only runtime for client-side web applications and continues to gain powerful APIs (WebGPU, WebTransport, WebAssembly).