JS Promise and microtask queue #102

Implement the Promise built-in and the microtask queue for asynchronous execution.

Scope#

Promise#

• Promise(executor) constructor — executor receives (resolve, reject)
• Promise states: pending, fulfilled, rejected
• Promise.prototype.then(onFulfilled, onRejected) — returns new Promise
• Promise.prototype.catch(onRejected) — sugar for then(undefined, onRejected)
• Promise.prototype.finally(onFinally) — runs callback regardless of outcome
• Promise resolution procedure (handle thenables)
• Promise.resolve(value) — wrap value in fulfilled promise (or return if already promise)
• Promise.reject(reason) — return rejected promise
• Promise.all(iterable) — fulfill when all fulfill, reject on first rejection
• Promise.allSettled(iterable) — wait for all to settle
• Promise.race(iterable) — settle with first settlement
• Promise.any(iterable) — fulfill with first fulfillment, reject with AggregateError if all reject

Microtask Queue#

• Job queue for promise reactions (then/catch/finally callbacks)
• Microtasks run to completion after each macrotask
• Ordering: microtasks drain fully before next macrotask
• Integration point with VM event loop (prepare for setTimeout in Phase 11)

Execution Model#

• Promise reactions enqueue microtasks (not executed synchronously)
• Chained promises propagate values/errors through the chain
• Unhandled rejection tracking (basic — log or flag)

Acceptance Criteria#

• Promise constructor calls executor synchronously
• then/catch/finally return new Promises with correct chaining
• Promise.resolve/reject create correctly-stated promises
• Promise.all fulfills with array of results
• Promise.all rejects on first rejection
• Promise.race settles with first settlement
• Promise.allSettled waits for all
• Promise.any rejects with AggregateError
• Microtask queue drains after synchronous code
• Chained promises propagate values correctly
• Unit tests for all Promise methods and chaining scenarios

Phase 10 — JavaScript Engine (issue 13 of 15). Depends on: JS core built-ins.