The JavaScript engine

The engine is five modules: jslex (tokens), jsparse (syntax tree), jsvalue (values and objects), jsregex (regular expressions) and jsinterp (the evaluator). jsdom then binds the DOM in as host objects.

Design

It is a tree-walking interpreter. That keeps it small and makes it easy to inspect and patch, at the cost of speed and of a few language features that need a suspendable stack (see below).

Realms

Each frame gets its own realm. A script inside an iframe sees that frame's window, document and globals, while the built-in intrinsics (Object, Array, Error...) are shared with the main realm through the prototype chain. Name lookup resolves against the realm's global at the root of the scope chain, so an unbound identifier in a frame finds the frame's globals first. This one change fixed a whole directory of attribute tests, because many of them run the same checks in an iframe.

Language features

Implemented: functions and closures, classes, let/const, destructuring, spread, template literals, getters and setters, Proxy, Reflect, Object.defineProperties, symbols, regular expressions and the usual built-ins. Prototype rules follow the spec, including __proto__ accessors and the immutable-prototype behaviour of the global object.

Host hooks

• hostIndexer: lets the DOM expose live indexed access (list[i]) on collections without copying them into arrays.
• Reflection: one generic accessor pair serves every reflected attribute rather than thousands of hand-written getters.
• Event handler attributes are compiled on first use and cloned with the node.

Known gaps

• await cannot suspend and generators are unimplemented, because a tree-walking evaluator lives on the native stack. A bytecode VM would fix both and is the largest single lever left.
• No JIT. Speed is that of an interpreter.
• Out of memory is not recoverable. The runtime's allocator can fail by emptying a sequence and carrying on, so a realm that runs out of memory can crash on its next index. The runner fences steps, depth and memory to keep this out of results.