TransformStream

A TransformStream is a combination of a ReadableStream in a WritableStream. It can be added after any ReadableStream using it’s pipeThrough method. The pipeThrough will return another ReadableStream that can be used for chaining.

+----------------+                   +-----------------+              +----------------+
| ReadableStream | ---pipeThrough--> | TransformStream | ---pipeTo--> | WritableStream |
+----------------+                   +-----------------+              +----------------+

const reader = new ReadableStream<number>({
  start(controller) {
    controller.enqueue(1)
    controller.enqueue(2)
    controller.enqueue(3)
    controller.close()
  },
})

const transformer = new TransformStream<number, string>(
  transform(chunk, controller) {
    controller.enqueue(chunk.toString())
  }
)

const writer = new WritableStream<string>({
  write(chunk) {
    console.info(chunk)
  },
})

reader.pipeThrough(transformer).pipeTo(writer).then(() => console.info('done'))
// '1'
// '2'
// '3'
// 'done'

If the transform function were to return a Promise, then it won’t be called again until the Promise resolves.