Jasper - Experimental JS to WASM Compiler

Overview

Jasper is an experimental compiler designed to translate JavaScript code directly into WebAssembly (WASM). This ambitious project explores the challenges and possibilities of ahead-of-time (AOT) compilation for JavaScript, a language traditionally interpreted or just-in-time compiled.

The project is currently in early development stages, achieving less than 5% compatibility with the Test262 test suite. Despite this early stage, it represents a significant exploration into the complexities of JavaScript compilation and WebAssembly generation.

Compilation Approaches

The project investigates three potential approaches for compiling JavaScript to WASM:

1. Direct Core WASM Instructions (Current Focus)

Compiling JavaScript directly to core WebAssembly instructions. This approach offers:

• Maximum Control: Direct mapping of JavaScript semantics to WASM
• Performance Potential: Optimal code generation without intermediate layers
• Complexity: Requires implementing all JavaScript features from scratch

2. WASM Component Model

Leveraging the emerging WebAssembly Component Model for modular compilation:

• Modularity: Better code organization and reusability
• Interoperability: Enhanced integration with other WASM components
• Future-Proof: Aligned with WASM’s evolution direction

3. QuickJS Bytecode Translation

Converting JavaScript to QuickJS bytecode, then translating to WASM:

• Proven Foundation: Builds on QuickJS’s mature JavaScript implementation
• Faster Development: Leverages existing JavaScript semantics
• Performance Trade-off: Additional abstraction layer

Technical Implementation

OXC Integration

The compiler uses OXC (Oxc Parser) for JavaScript parsing:

• High Performance: Rust-based parser with excellent speed
• Standards Compliance: Full ECMAScript specification support
• Error Recovery: Robust handling of malformed JavaScript

Custom Memory Management

One of the most challenging aspects has been implementing memory management:

• Custom Allocator: Built from scratch for WASM linear memory model
• Garbage Collection: Exploring mark-and-sweep and reference counting approaches
• Memory Safety: Ensuring no memory leaks or invalid access patterns

Architecture

┌─────────────────┐    ┌─────────────────┐    ┌─────────────────┐
│   JavaScript    │    │   OXC Parser    │    │      AST        │
│   Source Code   │───▶│   (Tokenizer)   │───▶│   Analysis      │
└─────────────────┘    └─────────────────┘    └─────────────────┘
                                                        │
                                                        ▼
┌─────────────────┐    ┌─────────────────┐    ┌─────────────────┐
│     WASM        │    │   Code Gen      │    │   IR Builder    │
│    Binary       │◀───│   (Backend)     │◀───│  (Frontend)     │
└─────────────────┘    └─────────────────┘    └─────────────────┘

Current Features

The compiler currently supports a minimal subset of JavaScript:

Control Flow

• Loops: for, while, and do-while constructs
• Conditionals: if/else statements with proper branching
• Basic Blocks: Proper control flow graph generation

Data Operations

• Variable Assignments: Local and global variable handling
• Primitive Types: Numbers, booleans, and basic string operations
• Basic Arithmetic: Mathematical operations with proper type coercion

Memory Management

• Linear Memory: WASM linear memory allocation and deallocation
• Stack Management: Function call stack and local variables
• Basic GC: Simple mark-and-sweep garbage collection

Technical Challenges

JavaScript Semantics Complexity

JavaScript’s dynamic nature presents unique compilation challenges:

• Dynamic Typing: Runtime type checking and coercion
• Prototype Chain: Object inheritance and property lookup
• Hoisting: Variable and function declaration behavior
• Closures: Lexical scoping and closure capture

WebAssembly Limitations

WASM’s design constraints require creative solutions:

• Linear Memory Model: Implementing JavaScript’s object model
• No Built-in GC: Custom garbage collection implementation
• Limited Types: Mapping JavaScript’s rich type system
• Stack Machine: Translating to WASM’s stack-based execution

Performance Considerations

Balancing correctness with performance:

• Code Size: Minimizing generated WASM binary size
• Runtime Performance: Optimizing hot code paths
• Startup Time: Fast module instantiation and initialization

Development Insights

Building this compiler has provided deep insights into:

1. Compiler Design: Frontend/backend separation and IR design
2. Memory Allocators: Custom allocation strategies for constrained environments
3. WebAssembly Internals: Deep understanding of WASM specification
4. JavaScript Semantics: Comprehensive knowledge of ECMAScript behavior

Goals and Roadmap

Short-term Goals

• Test262 Compatibility: Achieve 50% compatibility with Test262 test suite
• Core Features: Implement functions, objects, and arrays
• Error Handling: Proper exception handling and error propagation

Medium-term Goals

• WASI Integration: File system and system call support
• Optimization Passes: Dead code elimination and constant folding
• Component Model: Experiment with WASM component model approach

Long-term Vision

• Production Ready: Reliable compilation for real-world JavaScript
• Performance Competitive: Match or exceed V8/SpiderMonkey performance
• Ecosystem Integration: NPM package and toolchain integration

Learning Resources

The project draws inspiration from extensive research:

• Memory Management: Custom WASM allocators and GC strategies
• Compiler Theory: Frontend design and code generation techniques
• WebAssembly Spec: Deep dive into WASM instruction set and semantics
• JavaScript Engines: Study of V8, SpiderMonkey, and QuickJS implementations

Results and Impact

While still experimental, Jasper demonstrates:

• Technical Feasibility: JavaScript-to-WASM compilation is possible
• Learning Value: Deep understanding of both JavaScript and WebAssembly
• Research Contribution: Exploration of novel compilation approaches
• Open Source: Contributing to the WASM/JavaScript compilation research

Future Enhancements

• Advanced Optimizations: Inlining, loop optimization, and dead code elimination
• Debugging Support: Source maps and debugging information generation
• Module System: ES6 modules and CommonJS support
• Standard Library: Comprehensive JavaScript standard library implementation
• Performance Profiling: Detailed performance analysis and optimization tools

This project represents an ambitious exploration of the boundaries between JavaScript and WebAssembly, pushing the limits of what’s possible in ahead-of-time JavaScript compilation.

• Export Functionality: Save and load processor states