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  3. Node.js — What is `Symbol.iterator`?

Node.js — What is `Symbol.iterator`?

JavaScript has support for iteration protocols. These iteration protocols aren’t built-in implementations, but protocols outlining the convention that an implementation needs to follow. The Iteration protocol allows custom JavaScript objects to provide iteration behavior, for example how they provide values when looping over them in a for…of loop.

A JavaScript object is iterable when it has an @@iterator property. This @@iterator key is available via the global Symbol.iterator constant.

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Using Symbol.iterator in Your Code

The Symbol.iterator is a static property. It represents the well-known Symbol@@iterator. JavaScript looks up this property when iterating through an object and uses the provided iterator to retrieve the values.

You can implement your iterators using the Symbol.iterator constant using an ordinary function or a generator function. Let’s look at both ways in the following sections.

Iterator From an Ordinary Function

Here’s a simple implementation of a custom iterable from an ordinary function. You’re implementing a function using Symbol.iterator and providing the return values based on the iteration protocols.

let counter = 0

const myIterable = {  
  [Symbol.iterator] () {
    return this
  },

  next () {
    counter += 1

    return counter <= 3
      ? { done: false, value: counter }
      : { done: true, value: undefined }
  }
}

console.log([...myIterable]);  
// [1, 2, 3]

Notice the next() function in the iterable. You need this function to follow the iteration protocol. Iterables from ordinary functions must provide a next() function that returns the current iteration value or a marker that all values or consumed.

Iterator From a Generator Function

You can also use generator functions for iterables. Use the yield keyword within the generator function to provide each value:

const myIterable = {  
  * [Symbol.iterator] () {
    yield 1
    yield 2
    yield 3
  }
}

console.log([...myIterable]);  
// [1, 2, 3]

That’s it!

We’ll implement a full iterator as a class in the next tutorial.

Mentioned Resources

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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