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.

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 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 – 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
• 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).

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 or an explicit error will be thrown.

Promises that are synchronously resolved can't be chained

Throwing an exception directly in the promise body counts as a synchronous resolution, and the error will therefore not be caught.

new SyncPromise(function(resolve, reject) {
  resolve('foo'); // <- Sync resolve
}).then(function() {
  // Bad! This is disallowed, error will be thrown
});

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:

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.

var 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

npm install sync-promise

Then:

var SyncPromise = require('sync-promise');

Browser

bower install sync-promise

Then include the global export or the AMD module.

Example

// 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) {
    var req = IDBStore.get(key);
    req.onsuccess = function() {
      if (req.result !== undefined) {
        resolve(req.result);
      } else {
        reject('KeyNotFoundError');
      }
    };
    req.onerror = reject;
  });
}

// Usage
var tx = db.transaction('books', 'readonly');
var 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.
• SyncPromise.race only allows promises in its array argument and SyncPromise.all must have at least one promise in its array argument. These requirements are to avoid synchronous resolution of the promise these methods are to return.
• Promise.resolve and Promise.reject are not implemented – they don't make sense given the above restrictions

API

new SyncPromise(function)

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

Example:

var p = new SyncPromise(function(resolve, reject) {
  var 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:

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:

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. Note that, unlike for ES6 promises, at least one of the supplied values in the array must be a promise.

Example:

var 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:

var ps = [
  new SyncPromise(function(resolve) {
    setTimeout(function() {
      resolve(1);
    }, 100);
  }),
  new SyncPromise(function(resolve) {
    setTimeout(function() {
      resolve(2);
    }, 9);
  }),
];
SyncPromise.race(ps).then(function(ns) {
  assert.deepEqual(ns, 2);
});