Promise Reference Implementation

Reference implementation of ECMA-262 Promises, in JavaScript.

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This tries to stick as close as possible to the spec text but some things cannot be done with JavaScript...

1. CreateBuiltinFunction

When the spec wants us to create a function from some algorithm steps, instead we simply define an anonymous function with the steps.

Example

Solution:

let alreadyResolved = false;
const resolve = (0, (resolution) => {
  if (alreadyResolved) {
    return;
  }

  // ...
});

2. Completions

The spec really wants JavaScript to be a language where people use monads instead of exceptions. Unfortunately we can't really recreate this. Instead, we simply pull the completion out via a try-catch block.

Example

Solution:

// Let iteratorRecord be GetIterator(iterable).
// IfAbruptRejectPromise(iteratorRecord, promiseCapability).

// becomes:

let iteratorRecord;
try {
  iteratorRecord = GetIterator(iterable);
} catch (e) {
  iteratorRecord = e; // if iteratorRecord is an abrupt completion...

  // IfAbruptRejectPromise(iteratorRecord, promiseCapability)
  promiseCapability[kReject].call(undefined, iteratorRecord);
  return promiseCapability[kPromise];
}