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Streaming functionality in is built on the ES2018 AsyncIterator concept. This page gives some background on AsyncIterator since it is a recently introduced concept (at least as part of the JavaScript standard).


AsyncIterator is a standard JavaScript ES2018 feature and is well supported by recent evergreen browsers and Node.js versions.

The for await of iteration syntax is supported as well as the babel transpiler.

Batched Parsing and Endcoding using AsyncIterators

The input and output from streaming loaders and writers can both be expressed in terms of async iterators.

Using AsyncIterator

Remember tyhat an async iterator can be consumed (iterated over) via the for-await construct:

for await (const x of asyncIterable) {}

Using Streams as AsyncIterators

With a little effort, streams in JavaScript can be treated as AsyncIterators. As the section about Javascript Streams explains, instead of registering callbacks on the stream, you can now work with streams in this way:

for await (const buf of fs.createReadStream('foo.txt')) {
  // do something

Creating AsyncIterators

Remember that any object in JavaScript that implements the [Symbol.asyncIterator]() method is an AsyncIterable. And the async generator syntax can be used to generate new async iterators

async function* asyncIterator() {
  yield new Promise(...)

for await (const x of asyncIterator()) {} // Notice parens after 'asyncIterator'