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Undertanding JavaScript Iterables and Iterators

  1. Iteration
  2. Concept of Iterator
  3. Concept of Iterable

1. Iteration

ECMAScript 6 provides you with a new way to interact with data structure, which is Iteration. Now, we will clarify it.
There are two concepts that you need to distinguish:
  • Iterator
  • Iterable
The concepts of Iterator, Iterables apply to the Array, Set, and Map classes.

2. Concept of Iterator

Iterator: An object called Iterator if it contains a pointer to next element in Iteration.
Technically, an object in ECMAScript is called an Iterator if it has a method with the next() name, and this method returns an object with the form {value:SomeValue,done:booleanValue}. done has true value if the Iteration has been completed, otherwise it has false value.
iterator-object-example.js
// An Iterator Object:
let myIterator = { 
  someProp: "Some Prop",
  //
  next : function()  {
      return  {
         value: "Tom",
         done: false
      }
  }
}
// ----------- TEST -------------------
let entry = myIterator.next();
console.log(entry); // { value: 'Tom', done: false }
console.log(entry.value); // Tom

3. Concept of Iterable

Iterable - In terms of language, an object is called Iterable if it contains a collection of data, and it provides a way to publicize its data. For example, an array can be called Iterable because it contains a collection of data, and you can access its elements.
Technically, in the ECMAScript, an object that is called Iterable must have a method with the key of Symbol.iterator and this method returns an Iterator object
Symbol.iterator is a value of the Symbol data type, like 100 is a value of Integer type. You can see also Symbol in the following post:
Below is a simple example, an object with properties:
  • myProp1
  • myProp2
  • 100
  • myProp3
  • Symbol.iterator
object-with-properties.js
// An Object:
let myObject = {
   // A property
   myProp1 : "Some Value 1",
    // A property
   'myProp2' : "Some Value 2",
   // A property
   100 : "One hundred",
   // A property (Method)
   myProp3 :  function()  {
       console.log("I'm a method");
   },
   // A property (Method)
   [Symbol.iterator] : function() {
      console.log("I'm a [Symbol.iterator] method");
   }
} 
// ----------- TEST --------------
console.log( myObject["myProp1"] ); // Some Value 1
console.log( myObject["myProp2"] ); // Some Value 2
console.log( myObject[100] ); // One hundred
myObject["myProp3"](); // I'm a method
myObject[Symbol.iterator]();// I'm a [Symbol.iterator] method
Output:
Some Value 1
Some Value 2
One hundred
I'm a method
I'm a [Symbol.iterator] method
Iterable?
To sum up, an object is called Iterable if it has a method with a Symbol.iterable key, and this method returns an Iterator object.
iterable-object-example.js
// An Iterator Object
let myIterator = {
   next : function()  {
      return  {value: Math.random(), done: false};
   }
}
// A Iterable object:
let myIterable = {
   myProp : "Some value",
   // A Method returns an Iterator object.
   [Symbol.iterator] :  function() {
       return myIterator;
   }
}
// ------ TEST -----------
// An iterator object.
let it = myIterable[Symbol.iterator]();

let entry = it.next();
console.log(entry);

entry = it.next();
console.log(entry);
Output:
{ value: 0.6026736348993575, done: false }
{ value: 0.1790056262472559, done: false }
A class is called Iterable if it has a method with the [Symbol.iterator] name. The objects created from this class will be Iterable objects.
class-iterable-example.js
// An Iterable class.
class MyClass {
    constructor() { 
    }
    someMethod() {
        console.log("Some method");
    }
    [Symbol.iterator]() {
        // An Iterator object.
        var myIterator = {
            next: function() {
                return {value: Math.random(),done: false};
            }
        };
        return myIterator;
    }
}
// ----------- TEST -----------------
let myObject = new MyClass();
myObject.someMethod();
// An Iterator Object.
let it = myObject[Symbol.iterator]();
let entry;
let i = 0;
while ( (entry = it.next()).done == false )  {
    console.log( entry);
    i++;
    if(i > 5)  {
      break;
    }
}
Note: In the ECMAScript, the Array, Set, Map classes are Iterable.

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