How to make an iterable object in JavaScript?

Published August 14, 2020

What is an Iterable in JavaScript?

Iterable is a fancy word for those objects which can be looped or iterated in JavaScript.

Some of the common iterables we use in JavaScript are:

  • Strings
  • Arrays
  • Maps
  • Sets

etc.

These iterables if we put into a for..of looping statement just loops without any manual work from the programmer.

For example,

// a simple array
const arr = [1, 2, 3, 4];

// looping using for...of looping statement
// beacuse array is an iterable
for (let element of arr) {
  console.log(element);
}

The for...of looping statement can do that looping only on iterables. Not on ordinary objects.

How to make an Iterable Object in JavaScript?

  • To make an object iterable you have to attach a special method called Symbol.iterator.

  • This method should return an object with a valid next() method, this method is used by the for...of looping statement.

  • The next() method should also return the value in { done: false, value: value } format.

    • The done property refers to let know that whether the iteration has ended.
    • The value property should contain the latest value which is getting looped.

Let's understand this with the help of an example.

Suppose we have an ordinary object called oneToTen with two properties start and end which represents the starting of the loop value and ending of the loop value respectively.

We only have these two properties and we want to loop from 1 to 10 using the for...of looping statement instead of writing every value ourselves.

Fro that we need to make this object into an iterable object.

// oneToTen object
const oneToTen = {
  start: 1,
  end: 10,
};

Now Let make the special method Symbol.iterator to make it an iterable object.

Let's add the Symbol.iterator method using the bracket notation.

// oneToTen object
const oneToTen = {
  start: 1,
  end: 10,
};

// adding Symbol.iterator to oneToTen object using
// the bracket notation.
oneToTen[Symbol.iterator] = function () {
  // we need to return an object
  // containing a valid next() method
};

You need to return an object with a valid next() method from the Symbol.iterator special method.

// oneToTen object
const oneToTen = {
  start: 1,
  end: 10,
};

// adding Symbol.iterator to oneToTen object using
// the bracket notation.
oneToTen[Symbol.iterator] = function () {
  // we need to return an object
  // containing a valid next() method
  return {
    next() {
      // the next() method is automatically called
      // by the for..of looping statement
    },
  };
};

This next() method is automatically invoked by the for...of looping statement. So that the next() needs to return an object in the format {done: true | false, value: loopingValue}.

Inside this next() method is where we need to have our iterator logic of incrementing the looping value by 1.

Let's write that logic and return looping value in the above format of .{done: true | false, value: loopingValue}.

// oneToTen object
const oneToTen = {
  start: 1,
  end: 10,
};

// adding Symbol.iterator to oneToTen object using
// the bracket notation.
oneToTen[Symbol.iterator] = function () {
  // we need to return an object
  // containing a valid next() method
  let current = this.start;
  let last = this.end;
  return {
    next() {
      // the next() method is automatically called
      // by the for..of looping statement

      // checking is current is less than last
      if (current <= last) {
        return {
          done: false,
          value: current++, // incrementing current value by one
        };
      } else {
        // otherwise return done:true
        // to let know that looping has ended
        return {
          done: true,
        };
      }
    },
  };
};

Now we have defined our iterator logic.

Let's run it with the for...of looping statement.

// oneToTen object
const oneToTen = {
  start: 1,
  end: 10,
};

// adding Symbol.iterator to oneToTen object using
// the bracket notation.
oneToTen[Symbol.iterator] = function () {
  // we need to return an object
  // containing a valid next() method
  let current = this.start;
  let last = this.end;
  return {
    next() {
      // the next() method is automatically called
      // by the for..of looping statement

      // checking is current is less than last
      if (current <= last) {
        return {
          done: false,
          value: current++, // incrementing current value by one
        };
      } else {
        // otherwise return done:true
        // to let know that looping has ended
        return {
          done: true,
        };
      }
    },
  };
};

// looping using for...of
for (let value of oneToTen) {
  console.log(value);
}

The Process here is as follows

  • The for...of looping statement automatically calls the next() method from the object returned by the special Symbol.iterator method.
  • The next() method returns the current looping value in a {done: false, value: loopingValue} format.
  • The for...of looping statement automatically extracts the value from the value property from the object.

The output is:

1
2
3
4
5
6
7
8
9
10

We have made our ordinary object into a valid iterable object.

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