# synchronous-promise
TL;DR: A prototypical animal which looks like an A+ Promise but doesn't defer
immediately, so can run synchronously, for testing. Technically, this makes it
*not* A+ compliant, since part of the A+ spec is that resolution be asynchronous.
This means that I unfortunately can't run the official tests at [https://github.com/promises-aplus/promises-tests](https://github.com/promises-aplus/promises-tests). As such, I rely on issue reports from users and welcome contributions.
If `synchronous-promise` has made something easier for you and you'd like to say thanks,
[check out my sponsors page](https://github.com/sponsors/fluffynuts).
### Why?
The standard ES6 Promise (and any others which *are* A+ compliant) push the promise logic to the background
immediately, departing from the mechanisms employed in years past by promise
implementations in libraries such as jQuery and Q.
This is a good thing -- for production code. But it can make testing more
convoluted than it really needs to be.
Often, in a test, we're stubbing out a function which would return a promise
(eg http call, show a modal dialog requiring user interaction) with a promise
that resolves immediately, eg (using, mocha/sinon/chai):
```javascript
describe('the thing', () => {
it('will do some stuff', () => {
// Arrange
const asyncLibraryFake = {
someMethod: sinon.stub().returns(Promise.resolve('happy value!'))
},
sut = createSystemUnderTestWith(asyncLibraryFake);
// Act
sut.doSomethingInteresting();
// Assert
// [*]
})
});
```
[*] Ideally, we'd just have assertions here, but the code above has backgrounded,
so we're not going to get our expected results unless we employ async testing
strategies ourselves.
One strategy would be to return the promise from
asyncLibraryFake.someMethod
from the
doSomethingInteresting
function and perform our asserts in there:
```javascript
describe('the thing', () => {
it('will do some stuff', done => {
// Arrange
const asyncLibraryFake = {
someMethod: sinon.stub().returns(Promise.resolve('happy value!'))
},
sut = createSystemUnderTestWith(asyncLibraryFake);
// Act
sut.doSomethingInteresting().then(() => {
// Assert
done()
});
})
});
```
***And there's nothing wrong with this strategy.***
I need to put that out there before anyone takes offense or thinks that I'm suggesting
that they're "doing it wrong".
If you're doing this (or something very similar), great; `async/await`, if available,
can make this code quite clean and linear too.
However, when we're working on more complex interactions, eg when we're not
testing the final result of a promise chain, but rather testing a side-effect
at some step during that promise chain, this can become more effort to test
(and, imo, make your testing more unclear).
Many moons ago, using, for example, Q, we could create a deferred object with
`Q.defer()` and then resolve or reject ith with `deferred.resolve()` and
`deferred.reject()`. Since there was no initial backgrounding, we could set
up a test with an unresolved promise, make some pre-assertions, then resolve
and make assertions about "after resolution" state, without making our tests
async at all. It made testing a little easier (imo) and the idea has been
propagated into frameworks like `angular-mocks`
### Usage
SynchronousPromise looks (from the outside) a lot like an ES6 promise. We construct
the same:
```javascript
var promise = new SynchronousPromise((resolve, reject) => {
if (Math.random() < 0.1) {
reject('unlucky!');
} else {
resolve('lucky!');
}
});
```
They can, of course, be chained:
```javascript
var initial = new SynchronousPromise((resolve, reject) => {
resolve('happy!');
});
initial.then(message => {
console.log(message);
})
```
And have error handling, either from the basic A+ spec:
```javascript
initial.then(message => {
console.log(message);
}, error => {
console.error(error);
});
```
Or using the more familiar `catch()`:
```javascript
initial.then(message => {
console.log(message);
}).catch(error => {
console.error(error);
})
```
`.catch()` starts a new promise chain, so you can pick up with new logic
if you want to. `.then()` can deal with returning raw values or promises
(as per A+)
`SynchronousPromise` also supports `.finally()` as of version 2.0.8.
### Statics
`.all()`, `.resolve()` and `.reject()` are available on the `SynchronousPromise`
object itself:
```javascript
SynchronousPromise.all([p1, p2]).then(results => {
// results is an array of results from all promises
}).catch(err => {
// err is any single error thrown by a promise in the array
});
SynchronousPromise.resolve('foo'); // creates an already-resolved promise
SynchronousPromise.reject('bar'); // creats an already-rejected promise
```
(`race()` isn't because I haven't determined a good strategy for that yet,
considering the synchronous design goal -- but it's
unlikely you'll need `race()` from a test).
### Extras
`SynchronousPromise` also provides two extra functions to make testing a little
easier:
#### Static methods
The `unresolved()` method returns a new, unresolved `SynchronousPromise` with
the constructor-function-provided `resolve` and `reject` functions attached as properties.
Use this when you have no intention of resolving or rejecting the promise or when you
want to resolve or reject at some later date.
```javascript
var
resolvedValue,
rejectedValue,
promise = SynchronousPromise.unresolved().then(function(data) {
resolvedValue = data;
}).catch(function(data) {
rejectedValue = data;
});
// at this point, resolved and rejected are both undefined
// ... some time later ...
if (Math.random() > 0.5) {
promise.resolve("yay");
// now resolvedValue is "yay" and rejectedValue is still undefined
} else {
promise.reject("boo");
// now rejectedValue is "boo" and resolvedValue is still undefined
}
```
#### Instance methods
`pause()` pauses the promise chain at the point at which it is called:
```javascript
SynchronousPromise.resolve('abc').then(data => {
// this will be run
return '123';
}).pause().then(data2 => {
// we don't get here without resuming
});
```
and `resume()` resumes operations:
```javascript
var
promise = SynchronousPromise.resolve('123').pause(),
captured = null;
promise.then(data => {
captured = data;
});
expect(captured).to.be.null; // because we paused...
promise.resume();
expect(captured).to.equal('123'); // because we resumed...
```
You can use `pause()` and `resume()` to test the state of your system under
test at defined points in a series of promise chains
### ES5
SynchronousPromise is purposefully written with prototypical, ES5 syntax so you
can use it from ES5 if you like. Use the `synchronous-promise.js` file from the
`dist` folder if you'd like to include it in a browser environment (eg karma).
## Typescript
The `synchronous-promise` package includes an `index.d.ts`. To install, run:
```
typings install npm:synchronous-promise --save
```
*On any modern TypeScript (v2+), you shouldn't need to do this.*
Also note that TypeScript does async/await cleverly, treating all promises
equally, such that `await` will work just fine against a SynchronousPromise -- it just won't be backgrounded.
**HOWEVER:** there is a _very specific_ way that SynchronousPromise
can interfere with TypeScript: if
- SynchronousPromise is installed globally (ie, overriding the
native `Promise` implementation) and
- You create a SynchronousPromise which is resolved asynchronously,
eg:
```js
global.Promise = SynchronousPromise;
await new SynchronousPromise((resolve, reject) => {
setTimeout(() => resolve(), 0);
}); // this will hang
```
This is due to how TypeScript generates the `__awaiter` function
which is `yielded` to provide `async`/`await` functionality, in
particular that the emitted code assumes that the global `Promise`
will _always be asynchronous_, which is normally a reasonable assumption.
Installing SynchronousPromise globally may be a useful testing tactic,
which I've used in the past, but I've seen this exact issue crop up
in production code. `SynchronousPromise` therefor also provides two methods:
- `installGlobally`
- `uninstallGlobally`
which can be used if your testing would be suited to having `Promise` globally
overridden by `SynchronousPromise`. This needs to get the locally-available `__awaiter` and the result (enclosed with a reference to the real `Promise`)
must override that `__awaiter`, eg:
```ts
declare var __awaiter: Function;
beforeEach(() => {
__awaiter = SynchronousPromise.installGlobally(__awaiter);
});
afterEach(() => {
SynchronousPromise.uninstallGlobally();
});
```
It's not elegant that client code needs to know about the transpiled
code, but this works.
I have an issue open on GitHub
[https://github.com/Microsoft/TypeScript/issues/19909](https://github.com/Microsoft/TypeScript/issues/19909)
but discussion so far has not been particularly convincing that
TypeScript emission will be altered to (imo) a more robust
implementation which wraps the emitted `__awaiter` in a closure.
### Production code
The main aim of SynchronousPromise is to facilitate easier testing. That being
said, it appears to conform to expected `Promise` behaviour, barring the
always-backgrounded behaviour. One might be tempted to just use it everywhere.
**However**: I'd highly recommend using *any* of the more venerable promise implementations
instead of SynchronousPromise in your production code -- preferably the vanilla
ES6 Promise, where possible (or the shim, where you're in ES5). Or Q.
Or jQuery.Deferred(), Bluebird or any of the implementations at [https://promisesaplus.com/implementations](https://promisesaplus.com/implementations).
Basically, this seems to work quite well for testing and
I've tried to implement every behaviour I'd expect from a promise -- but I'm
pretty sure that a native `Promise` will be better for production code any day.
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