JustScript

A lightweight JavaScript engine written in Rust, designed from the ground up to support three execution modes:

• Interpreter — a stack-based bytecode virtual machine.
• JIT — just-in-time native compilation via Cranelift.
• AOT — ahead-of-time compilation to native object files via Cranelift.

Status: early scaffold. The module layout and public APIs are in place; individual passes are being filled in incrementally.

Architecture

JustScript is organized as a Cargo workspace of focused crates, with strict acyclic dependencies (js-syntax is the leaf, js-cli is the root):

                  js-syntax  ◀──── shared by almost every crate
                     ▲
        ┌────────────┼─────────────┐
   js-lexer   js-diagnostics   js-runtime
        ▲          ▲               ▲
        └────┐     │      ┌────────┘
         js-parser │      │
              ▲    │      │
            js-bytecode
                 ▲
        ┌────────┴────────┐
      js-vm          js-codegen  (Cranelift, features: jit / aot)
                 ▲
             js-engine
                 ▲
              js-cli

Crate	Responsibility
js-syntax	Source positions, tokens, keywords, punctuators, AST.
js-lexer	Character stream cursor + tokenizer.
js-diagnostics	Span-aware diagnostics + DiagResult.
js-parser	Recursive-descent + Pratt parser → AST.
js-runtime	Value, Object/Shape, GC, builtins, realm.
js-bytecode	Opcode set, bytecode functions, AST → bytecode.
js-vm	Bytecode interpreter / dispatch loop.
js-codegen	Cranelift JIT + AOT backends.
js-engine	Top-level Engine API, execution-mode pipeline.
js-cli	REPL + file runner (--interpret / --jit / --aot).

Usage

# Build the whole workspace
cargo build --workspace

# Run a file
cargo run -p js-cli -- run script.js --interpret

# Enable the JIT/AOT backends
cargo build --workspace --features js-codegen/jit --features js-codegen/aot

# Run the test suite
cargo test --workspace