https://github.com/devsnek/promise-reference

Reference implementation of ECMA-262 Promises, in JavaScript
https://github.com/devsnek/promise-reference

ecmascript promise reference-implementation

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Reference implementation of ECMA-262 Promises, in JavaScript

• Host: GitHub
• URL: https://github.com/devsnek/promise-reference
• Owner: devsnek
• Created: 2018-07-12T03:41:00.000Z (over 6 years ago)
• Default Branch: master
• Last Pushed: 2019-05-25T17:34:15.000Z (over 5 years ago)
• Last Synced: 2024-10-29T12:42:20.579Z (3 months ago)
• Topics: ecmascript, promise, reference-implementation
• Language: JavaScript
• Homepage:
• Size: 15.6 KB
• Stars: 3
• Watchers: 3
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md

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README

        
# Promise Reference Implementation

Reference implementation of ECMA-262 Promises, in JavaScript.

---

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](https://tc39.github.io/ecma262/#sec-promise-resolve-functions)

Solution:

```js

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](https://tc39.github.io/ecma262/#sec-promise.all)

Solution:

```js

// 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];

}

```