The Life of One JS Line — A Source-Level Field Map of QuickJS

This is an immersive long-form essay — best read in its dedicated layout → Open the full version

This is the seventh piece in the Field Note series, siblings to «How V8 makes JS fast» (multi-tier JIT) and «From Rust to SIMD» (WebAssembly). This one is the opposite — how a JIT-less JS engine ships full ES2023 in 70k lines of C, a 700 KB binary, and zero start-up cost. Source-level references track quickjs-ng main (compatible with Fabrice Bellard’s 2024-01-13 reference).

Five preparatory chapters + one through-line + fourteen pipeline stages + five synthesis chapters

We feed [1,2,3].map(x => x*2) to QuickJS and trace its byte stream end to end:

06 Lexer     07 Parser    08 AST→FuncDef  09 Bytecode
10 JSValue   11 Atom      12 Shape+Object 13 Closure   14 Class
15 Interp    16 Lookup    17 Promise/Gen  18 RegExp    19 GC

Every pipeline chapter carries a ”◇ In our JS line” input → output card anchoring its slice to the main line.

What this expansion adds

• Byte-level JSValue: 32-bit NaN-boxing vs 64-bit tagged, compared to V8’s Smi compression, JSC’s NaN-box, Hermes’s similar design.
• JSShape (hidden class): QuickJS has Shape but deliberately no inline caches — and that’s the core reason its hot path is 10-20× slower than V8.
• Interpreter main loop: 3000 lines of switch + computed goto with direct-threading tricks.
• Bytecode X-macros: one 250-opcode definition generating enum / dispatch table / metadata / emit helpers across 6 files.
• Closures: JSVarRef’s stack→heap transfer mechanism (borrowed from Lua’s upval).
• GC: refcount + trial-deletion cycle collection (same algorithm as Python / PHP).
• No JIT: four iron rules (single file, no JIT, refcount, no IC) make QuickJS 10-20× slower on hot paths but 30× faster startup, 20× smaller memory, 40× smaller binary.
• Embedding playbook: 30 lines of C to put JS into your project; JSRuntime vs JSContext realm model.
• Ecosystem map: QuickJS-ng / txiki.js / Just / early Cloudflare Workers / Android app scripting.
• Three-dimensional perf: across peak speed, startup, memory, QuickJS sits at the exact opposite corner from V8 / JSC / SpiderMonkey / Hermes.

Every chapter cites quickjs.c line ranges + key function names + key structs, plus the relevant ECMAScript spec § — so you can grep the source alongside this essay.

“In the world of JS engines, V8 will always be the F1 race car; QuickJS will always be the folding bicycle. The world needs both.”