Undertanding ECMAScript Iterables and Iterators

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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
 

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);
 
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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