## m.deferred

This is a low-level method in Mithril. It's a modified version of the Thenable API.

A deferred is an asynchrony monad. It exposes a `promise` property which can *bind* callbacks to build a computation tree.

The deferred object can then *apply* a value by calling either `resolve` or `reject`, which then dispatches the value to be processed to the computation tree.

Each computation function takes a value as a parameter and is expected to return another value, which in turns is forwarded along to the next computation function (or functions) in the tree.

The deferred object returned by `m.deferred` has two methods: `resolve` and `reject`, and one property called `promise`. The methods can be called to dispatch a value to the promise tree. The `promise` property is the root of the promise tree. It has a method `then` which takes a `successCallback` and a `errorCallback` callbacks. Calling the `then` method attaches the computations represented by `successCallback` and `errorCallback` to the promise, which will be called when either `resolve` or `reject` is called. The `then` method returns a child promise, which, itself, can have more child promises, recursively.

The `promise` object is actually a function - specifically, it's an [`m.prop`](mithril.prop.md) getter-setter, which gets populated with the value returned by  `successCallback` if the promise is resolved successfully.

Note that Mithril promises are not automatically integrated to its automatic redrawing system. If you wish to use third party asynchronous libraries (for example, `jQuery.ajax`), you should also consider using [`m.startComputation` / `m.endComputation`](mithril.computation.md) if you want views to redraw after requests complete.

---

### Usage

```javascript
//standalone usage
var greetAsync = function() {
	var deferred = m.deferred();
	setTimeout(function() {
		deferred.resolve("hello");
	}, 1000);
	return deferred.promise;
};

greetAsync()
	.then(function(value) {return value + " world"})
	.then(function(value) {console.log(value)}); //logs "hello world" after 1 second
```

---

#### Retrieving a value via the getter-setter API

```javascript
//asynchronous service
var greetAsync = function() {
	var deferred = m.deferred();
	setTimeout(function() {
		deferred.resolve("hello");
	}, 1000);
	return deferred.promise;
};

//asynchronous consumer
var greeting = greetAsync()
var processed = greeting.then(function(value) {return value + " world"})

console.log(greeting()) // undefined - because `deferred.resolve` has not been called yet

setTimeout(function() {
	//now `deferred.resolve` has been called
	console.log(greeting()) // "hello"
	console.log(processed()) // "hello world"
}, 2000)
```

---

#### Integrating to the Mithril redrawing system

```javascript
//asynchronous service
var greetAsync = function() {
	m.startComputation();

	var deferred = m.deferred();
	setTimeout(function() {
		deferred.resolve("hello");

		m.endComputation();
	}, 1000);
	return deferred.promise;
};
```

---

### Differences from Promises/A+

For the most part, Mithril promises behave as you'd expect a [Promise/A+](http://promises-aplus.github.io/promises-spec/) promise to behave, but has one difference:  
Mithril promises atempt to execute synchronously if possible.

To illustrate the difference between Mithril and A+ promises, consider the code below:

```javascript
var deferred = m.deferred()

deferred.promise.then(function() {
	console.log(1)
})

deferred.resolve("value")

console.log(2)
```

In the example above, A+ promises are required to log `2` before logging `1`, whereas Mithril logs `1` before `2`. Typically `resolve`/`reject` are called asynchronously after the `then` method is called, so normally this difference does not matter.

---

### Signature

[How to read signatures](how-to-read-signatures.md)

```clike
Deferred deferred()

where:
	Deferred :: Object { Promise promise, void resolve(any value), void reject(any value) }
	Promise :: GetterSetter { Promise then(any successCallback(any value), any errorCallback(any value)) }
	GetterSetter :: any getterSetter([any value])
```

-	**GetterSetter { Promise then([any successCallback(any value) [, any errorCallback(any value)]]) } promise**

	A promise has a method called `then` which takes two computation callbacks as parameters.

	The `then` method returns another promise whose computations (if any) receive their inputs from the parent promise's computation.

	A promise is also a getter-setter (see [`m.prop`](mithril.prop.md)). After a call to either `resolve` or `reject`, it holds the result of the parent's computation (or the `resolve`/`reject` value, if the promise has no parent promises)

	-	**Promise then([any successCallback(any value) [, any errorCallback(any value)]])**

		This method accepts two callbacks which process a value passed to the `resolve` and `reject` methods, respectively, and pass the processed value to the returned promise

		-	**any successCallback(any value)** (optional)

			The `successCallback` is called if `resolve` is called in the root `deferred`.

			The default value (if this parameter is falsy) is the identity function `function(value) {return value}`

			If this function returns undefined, then it passes the `value` argument to the next step in the thennable queue, if any

		-	**any errorCallback(any value)** (optional)

			The `errorCallback` is called if `reject` is called in the root `deferred`.

			The default value (if this parameter is falsy) is the identity function `function(value) {return value}`

			If this function returns undefined, then it passes the `value` argument to the next step in the thennable queue, if any

		-	**returns Promise promise**

-	**void resolve(any value)**

	This method passes a value to the `successCallback` of the deferred object's child promise

-	**void reject(any value)**

	This method passes a value to the `errorCallback` of the deferred object's child promise