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  3. Check If a Value Is Async Iterable in JavaScript or Node.js

Check If a Value Is Async Iterable in JavaScript or Node.js

JavaScript supports for await...of loops to iterate over async and sync iterables. The async iterator protocol has a similar interface as the synchronous iterator protocol. The difference is that the returned value from an async iterable is wrapped in a promise.

This tutorial shows you how to determine whether an object implements the async iterable protocols. We’re using JavaScript’s Symbol.asyncIterator for that check. Read on to learn more!

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Determine Whether a JavaScript Value is Async Iterable

JavaScript specifies async iterable protocols and how you’re walking through each of the provided values. To be async iterable, an object must implement the well-known @@asyncIterator symbol. Instead of hard-code that symbol in your source code, you may use the static constant Symbol.asyncIterator instead.

You can also use Symbol.asyncIterator to check whether a given input value is async iterable. If the implementation for the symbol is a function, it supports the async iterable protocol.

Check for the existence @@asyncIterator function and handle nullish values:

/**
 * Determine whether the given `input` is async iterable.
 *
 * @returns {Boolean}
 */
function isAsyncIterable(input) {  
  if (input == null) {
    return false
  }

  return typeof input[Symbol.asyncIterator] === 'function'
}

That’s it!

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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