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Chapter 9: Async Iterators
Introduction
An async function is one that returns a promise. await yields to the caller until the promise resolves
and then continues with the result.
An iterator allows the collection to be looped through with a for-of loop.
An async iterator is a collection where each iteration is a promise which can be awaited using a
for-await-of loop.
Async iterators are a stage 3 proposal. They are in Chrome Canary 60 with --harmony-async-
iteration
Syntax
• async function* asyncGenerator() {}
• yield await asyncOperationWhichReturnsAPromise();
• for await (let result of asyncGenerator()) { /* result is the resolved value from the promise */ }
Remarks
An async iterator is a declarative pull stream as opposed to an Observable's declarative push
stream.
Useful Links
• Async Iteration spec proposal
• Introduction to their use
• Event subscription proof of concept
Examples
Basics
A JavaScript Iterator is an object with a .next() method, which returns an IteratorItem, which is
an object with value : <any> and done : <boolean>.
A JavaScript AsyncIterator is an object with a .next() method, which returns a
Promise<IteratorItem>, a promise for the next value.
To create an AsyncIterator, we can use the async generator syntax:
/**
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