Ultra compact synchronized promise implementation.
npm install sync-promise-expandedA 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.
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.
* 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.
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).
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.
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 , however:
`javascript`
new SyncPromise(function(res, rej) {
setTimeout(function () {
throw new Error('err');
});
});
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;
`shell`
npm install sync-promise-expanded
Then:
`js
import SyncPromise from 'sync-promise-expanded';
SyncPromise.all([
// ...
]);
`
`shell`
npm install sync-promise-expanded
Then include the global export or the AMD module.
`javascriptresolve
// This is a wrapper around IDBStore#get.
// Had it been written using native promises it would have closed the
// transaction when calling 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.
});
* Synchronized resolution and rejection, of course.
* Promise.resolve and Promise.reject are implemented withsetTimeout(..., 0)
as are resolve() and reject() when run
synchronously.
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;
});
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);
});
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);
});
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]);
});
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);
});
Equivalent to:
`javascript`
return new SyncPromise(function(resolve, reject) {
setTimeout(function () {
resolve(val);
}, 0);
});
Equivalent to:
`javascript``
return new SyncPromise(function(resolve, reject) {
setTimeout(function () {
reject(val);
}, 0);
});