Is JavaScript compiled or interpreted? The answer is: both. Understanding the difference between compilers and interpreters helps you write faster code and use development tools more effectively.
What is a Compiler?
A compiler translates your entire source code into another form before execution. Traditional compilers convert high-level languages like C++ into machine code that runs directly on your CPU.
The key characteristics of compilation are:
- Ahead-of-time (AOT): Translation happens before runtime
- Whole-program analysis: The compiler sees all your code at once
- Optimization: Compilers can optimize code extensively
- Error checking: Many errors are caught before execution
For JavaScript, "compilation" usually means transforming modern syntax (ES2026, TypeScript, JSX) into browser-compatible JavaScript, and bundling multiple files into one.
What is an Interpreter?
An interpreter executes code line-by-line or statement-by-statement. It reads a piece of code, executes it immediately, then moves to the next piece.
Characteristics of interpretation:
- Just-in-time: Code runs as it's read
- No separate compilation step: You can run code instantly
- Runtime errors: Some errors only appear when code executes
- Flexibility: Great for dynamic, interactive environments
Early JavaScript engines were pure interpreters. You could type code in a browser console and see results immediately, which is why JavaScript became so popular for web development.
How JavaScript Actually Works
Modern JavaScript engines like V8 (Chrome, Node.js) and SpiderMonkey (Firefox) use a hybrid approach called Just-In-Time (JIT) compilation.
Here's what happens when you run JavaScript:
- Parsing: The engine parses your code into an Abstract Syntax Tree (AST)
- Baseline compilation: The AST is quickly compiled to bytecode or machine code
- Execution: The code starts running immediately
- Profiling: The engine monitors which code runs frequently ("hot" code)
- Optimization: Hot code is recompiled with aggressive optimizations
- Deoptimization: If assumptions are wrong, optimized code is thrown away
This hybrid approach gives you the best of both worlds: the instant startup of an interpreter and the performance of a compiler.
JavaScript Build Tools: Another Layer of Compilation
Beyond the browser's JIT compiler, modern JavaScript development often involves build-time compilation using tools like:
- Babel: Transpiles modern JavaScript to older versions
- TypeScript: Compiles TypeScript to JavaScript
- esbuild: Ultra-fast bundler and transpiler
- webpack/Rollup/Vite: Bundle modules and optimize code
These tools transform your source code before it reaches the browser. When you use TypeScript, for example, the TypeScript compiler checks types and outputs JavaScript. Then the browser's JIT compiler takes over.
Why PlayCode Uses Both
PlayCode's JavaScript compiler uses esbuild (compiled to WebAssembly) for build-time compilation. This handles:
- TypeScript and JSX transpilation
- Module bundling and tree-shaking
- npm package resolution
- Source map generation
The compiled bundle then runs in your browser, where the V8 engine's JIT compiler further optimizes the code at runtime.
This two-stage approach, build-time compilation plus runtime JIT, gives you instant feedback while coding (incremental builds in 20-50ms) and excellent runtime performance.
Compiler vs Interpreter: Key Differences
| Aspect | Compiler | Interpreter |
|---|---|---|
| When it runs | Before execution | During execution |
| Output | Executable file | Direct execution |
| Error detection | Many errors caught early | Errors at runtime |
| Startup speed | Slower (compilation time) | Faster (immediate start) |
| Runtime speed | Faster (optimized code) | Slower (no optimization) |
Practical Implications for Developers
Understanding this distinction helps you:
- Choose the right tools: Use TypeScript for type checking at compile time, catching errors before runtime
- Write optimizable code: Consistent types help JIT compilers optimize better
- Debug effectively: Know whether an error came from build-time compilation or runtime execution
- Optimize build times: Tools like esbuild are fast because they do less than traditional compilers
Try It Yourself
Want to see JavaScript compilation in action? Open PlayCode's JavaScript compiler and try writing some TypeScript or JSX. Watch the live preview update instantly as our compiler transforms your modern code into browser-ready JavaScript.
The AI assistant can even explain what's happening under the hood. Ask it "How does JavaScript compilation work?" and see the response.