javascriptcore-garbage-collector

Bun's JavaScriptCore garbage collector uses a non-moving, generational, parallel, mostly-concurrent design. It distinguishes Eden collections from Full GC cycles to manage memory performance across mutator and collector interactions.

Debug use-after-free bugs and JS object leaks by inspecting root tracing, WriteBarrier, and visitChildren constraints. Leverage HeapAnalyzer and analyzeHeap hooks to label, trace memory graphs, and capture heap snapshots.

Use weak references like JSC::Weak and WeakHandleOwner for JS↔native reference patterns in Zig bindings. WriteBarrier and visitAdditionalChildren keep objects alive across mutator and GC cycles.

IsoSubspace and opaque roots are core JavaScriptCore GC concepts. IsoSubspace manages specific object heap allocations, while opaque roots act as root constraints keeping objects alive during collection cycles.

JS object leaks after a Full GC cycle often occur when opaque roots or JSRef patterns in Zig bindings unintentionally keep references alive. Use HeapAnalyzer to trace the memory graph and identify lingering references.