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');
}).then(function() {
});
new SyncPromise(function(resolve, reject) {
setTimeout(resolve, 10);
}).then(function() {
return 1;
}).then(function(n) {
n === 1;
});
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);
});
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
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;
});
}
var tx = db.transaction('books', 'readonly');
var bookStore = tx.objectStore('books');
getRecord(bookStore, 'Bedrock Nights').then(function(book) {
});
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);
});