JS: Interface
New in JS2015.
Purpose of JavaScript Interface
JavaScript interface is the underlying mechanism for how Iterable Object works.
What is JavaScript interface?
An interface is a spec for a set of property keys and values .
Any object that has all the properties (or inherited) as described by an interface's spec, is said to have (conform to) that interface.
Note: by definition, many objects can have the same interface.
Note: by definition, a object can have many interfaces.
When a object confoms to interface X, we say that the object is βX objectβ. For example, a object that conforms to the iterable interface is called βiterable objectβ.
There are 3 interfaces defined by the JavaScript spec:
- Iterable interface
- Iterator interface
- IteratorResult interface
Iterable Interface
| Property | Value | Requirement |
|---|---|---|
Symbol.iterator [see Symbol Tutorial] | A function that returns a iterator object. | The returned object must conform to the Iterator interface. |
[see Iterable Object]
Iterator Interface
| Property | Value | Requirement |
|---|---|---|
"next" | A function that returns an IteratorResult object. | The returned object must conform to the IteratorResult interface. If a previous call to the "next" method of an Iterator has returned an IteratorResult object whose "done" property is true, then all subsequent calls to the "next" method of that object should also return an IteratorResult object whose "done" property is true. However, this requirement is not enforced.
|
The following are optional:
| Property | Value | Requirement |
|---|---|---|
"return" |
A function that returns an IteratorResult object. | The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller does not intend to make any more "next" method calls to the Iterator. The returned IteratorResult object will typically have a "done" property whose value is true, and a value property with the value passed as the argument of the return method. However, this requirement is not enforced. |
"throw" |
A function that returns an IteratorResult object. | The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to throw the value passed as the argument. If the method does not throw, the returned IteratorResult object will typically have a "done" property whose value is true. |
[see Iterator]
IteratorResult Interface
| Property | Value | Requirement |
|---|---|---|
"done" | true or false | This is the result status of an iterator next method call. If the end of the iterator was reached, "done" is true. If the end was not reached "done" is false and a "value" is available. If a "done" property (either own or inherited) does not exist, it is consider to have the value false. |
"value" | Any JavaScript value. | If "done" is false, this is the current iteration element value. If "done" is true, this is the return value of the iterator, if it supplied one. If the iterator does not have a return value, value is undefined. In that case, the value property may be absent from the conforming object if it does not inherit an explicit value property. |