# SyncPromise

A fast, small, _safe_ promise implementation with synchronous promise
resolution and an API which resembles ECMAScript promises.

SyncPromise is incompliant with the Promises/A+ spec, specifically part
[2.2.4](https://promisesaplus.com/#point-34).

## Why

Promises make handling asynchronous operations easier. IndexedDB exposes a
lot of asynchronous operations. That sounds like a great match? Well, [unfortunately things
are not so simple](http://stackoverflow.com/questions/28388129/inconsistent-interplay-between-indexeddb-transactions-and-promises/)
It is not possible to use Promises/A+ promises inside IndexedDB transactions
in a cross browser way.

SyncPromise was created because it's author wanted to use promises in
IndexedDB transaction for the library [SyncedDB](https://github.com/paldepind/synceddb)
– both internally and in the user facing API. It was released in the hope that
it would be of use to others who work directly with IndexedDB.

## Features

* Weighs less than 1KB when minified (not gzipped).
* Familiar API that is very similar to the native ECMAScript promises API.
* Provides a safety mechanism to prevent [releasing Zalgo](http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony)
* Distributed both as a CommonJS package, AMD module, global export and as a
  version suitable for including directly in other source code.

## Safety

It is for good reason that the Promises/A+ specification requires
asynchronous resolution! Without care taken one can end up creating promises
that are sometimes synchronous and sometimes asynchronous. That is a _very_
bad idea that leads to unpredictable non-deterministic behaviour ([see this post for a
detailed explanation](http://blog.ometer.com/2011/07/24/callbacks-synchronous-and-asynchronous/)).

### Restrictions

Fortunately SyncPromise imposes two restrictions on usage. The first ensures
that promises are never resolved immediately. The second makes sure
that no errors get swallowed. Together these restrictions ensure that a
promise chain will _always_ be run asynchronously.

### Promises that are synchronously resolved can't be chained

Throwing an exception directly in the promise body counts as a synchronous
resolution and will therefore be resolved instead with `setTimeout(..., 0)`.

```javascript
new SyncPromise(function(resolve, reject) {
  resolve('foo'); // <- Will be treated as async resolve
}).then(function(result) {
  result === 'foo'; // true
});

new SyncPromise(function(resolve, reject) {
  setTimeout(resolve, 10); // <- Asynchronous resolve
}).then(function() {
  return 1; // Fine!
}).then(function(n) {
  n === 1; // true
});
```

Uncaught errors will be thrown if the rejection occurs within the `SyncPromise`
function body and there is no `catch`, however:

```javascript
new SyncPromise(function(res, rej) {
  setTimeout(function () {
    throw new Error('err');
  });
});
```

### If a promise rejects, at least one `onRejected` callback must have been attached

This ensures that all rejected promises are handled. Other promise libraries
(Bluebird for instance) use async mechanisms to ensure this.

```javascript
const p = new SyncPromise(function(resolve, reject) {
  setTimeout(reject, 10); // Error is thrown – no rejection handlers attached yet
});
setTimeout(function() {
  p.catch(function() { });
}), 20;
```

## Installation

### Node.js/Browserify

```shell
npm install sync-promise-expanded
```

Then:

```js
import SyncPromise from 'sync-promise-expanded';

SyncPromise.all([
  // ...
]);
```

### Browser

```shell
npm install sync-promise-expanded
```

Then include the global export or the AMD module.

## Example

```javascript
// This is a wrapper around IDBStore#get.
// Had it been written using native promises it would have closed the
// transaction when calling `resolve` or `reject`
function getRecord(IDBStore, key) {
  return new SyncPromise(function(resolve, reject) {
    const req = IDBStore.get(key);
    req.onsuccess = function() {
      if (req.result !== undefined) {
        resolve(req.result);
      } else {
        reject('KeyNotFoundError');
      }
    };
    req.onerror = reject;
  });
}

// Usage
const tx = db.transaction('books', 'readonly');
const bookStore = tx.objectStore('books');

getRecord(bookStore, 'Bedrock Nights').then(function(book) {
  // We got the book, and the transaction is still open so we
  // can make another request. Had `getRecord` used native promises
  // the transaction whould have been closed by now.
});
```

## Differences from ECMAScript promises

* Synchronized resolution and rejection, of course.
* `Promise.resolve` and `Promise.reject` are implemented with
  `setTimeout(..., 0)` as are `resolve()` and `reject()` when run
  synchronously.

## API

### new SyncPromise(function)

Creates a new promise. The passed function is passed callbacks to both
resolve and reject the promise.

__Example:__

```javascript
const p = new SyncPromise(function(resolve, reject) {
  const req = IDBStore.get(key);
  req.onsuccess = function() {
    if (req.result !== undefined) {
      resolve(req.result);
    } else {
      reject('KeyNotFoundError');
    }
  };
  req.onerror = reject;
});
```

### SyncPromise#then(function)

The passed function will be called if the promise is fulfilled. A new promise
chained from the original promise is returned. The new promise is resolved with
the value that the function return. The new promise is rejected if the function
throws an error.

__Example:__

```javascript
getSomething.then(function(v) {
  return doSomething(v);
}).then(function(v) {
  doSomethingElse(v);
});
```

### SyncPromise#catch(function)

The passed function will be called if the promise is rejected. A new promise
chained from the original promise is returned. The new promise is resolved with
the value that the function return. The new promise is rejected if the function
throws an error.

__Example:__

```javascript
getSomething.then(function(v) {
  return doSomething(v);
}).then(function(v) {
  doSomethingElse(v);
});
```

### SyncPromise.all(array)

Return a promise that is resolved when all promises in the array has fulfilled.
If one rejects, the promise is rejected for the same reason.

__Example:__

```javascript
const ps = [
  new SyncPromise(function(resolve) {
    setTimeout(function() {
      resolve(1);
    }, 100);
  }),
  2,
  new SyncPromise(function(resolve) {
    setTimeout(function() {
      resolve(3);
    }, 9);
  }),
];
SyncPromise.all(ps).then(function(ns) {
  assert.deepEqual(ns, [1, 2, 3]);
});
```

### SyncPromise.race(array)

Return a promise that is resolved when one of the promises in the array has
fulfilled. If one rejects, the promise is rejected for the same reason.

__Example:__

```javascript
const ps = [
  new SyncPromise(function(resolve) {
    resolve(1);
  }),
  2,
  new SyncPromise(function(resolve) {
    setTimeout(function() {
      resolve(3);
    }, 9);
  }),
];
SyncPromise.race(ps).then(function(ns) {
  assert.deepEqual(ns, 2);
});
```

### SyncPromise.resolve(val)

Equivalent to:

```javascript
return new SyncPromise(function(resolve, reject) {
  setTimeout(function () {
    resolve(val);
  }, 0);
});
```

### SyncPromise.reject(val)

Equivalent to:

```javascript
return new SyncPromise(function(resolve, reject) {
  setTimeout(function () {
    reject(val);
  }, 0);
});
```