How to customize the JavaScript iterator
This article introduces the knowledge of "how to customize the JavaScript iterator". Many people will encounter this dilemma in the operation of actual cases, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!
1. what is an iterator?
Iterator is an interface mechanism that provides unified access to a variety of different data structures. (that is, to make some data structures that do not support traversal traversal)
The most common is the Array iterator, which returns values in an array sequentially.
Let arr = [1 console.log 2 3 4 5]; for (let val of arr) {console.log (val);}
2. custom iterator
So how do we implement iterators?
First of all, it needs to meet two points:
Iterable protocol
Iterator protocol
Iterable protocol
Iterable protocols allow JavaScript objects to define or customize their iterative behavior
How to satisfy the iterable protocol?
To become an iterable object, an object must implement the @ @ iterator method.
This means that an object (or an object on its prototype chain) must have a property with the key @ @ iterator, which can be accessed through the constant Symbol.iterator
That is to say, to satisfy the iterable protocol, your object needs to have a property named Symbol.iterator, which makes it an iterable object.
Iterator protocol
The iterator protocol defines a standard way to produce a series of values, whether finite or infinite.
When the value is limited and all values are iterated, a default return value is returned.
How to satisfy the iterator protocol?
Your object needs to implement at least one next () method, which returns an iterator object IteratorResult. This iterator object contains two properties, done and value.
Done:
False if the iterator can produce the next value in the sequence.
True if the iterator has finished iterating the sequence
Value
The value returned by the iterator. Omitted when done is true
So let's start customizing an iterator.
As mentioned earlier, to customize an iterator, you need to meet the following two conditions:
Become an iterable object, that is, have a Symbol.iterator property
(that is, iterative protocol: Symbol.iterator)
The iterator object returns a next () method, and the next () method returns an object containing value and done properties.
(i.e. iterator protocol: return {next () {return {value, done}})
Let colors = {blue: "blue", green: "green", yellow: "yellow"}
Colors is now a non-iterating object, and we want to use for... Of traverses it, so you can customize the iterator.
Next, start to implement:
Colors [Symbol.iterator] = function () {let keys = Object.keys (colors); / / if you use let keys = Reflect.ownKeys (colors), keys will include some attributes that cannot be enumerated / / then the following len should be subtracted by one, minus the attribute Symbol.iterator / / choose to use let len = keys.length; let index = 0 according to the actual situation Return {next: function () {if (index)
< len) { return { value : colors[keys[index++]], done : false } } return { done : true } } }} 让我们对其验证一下： for (let val of colors) { console.log(val);}
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