@johngw/stream/src/transformers/reduce.ts

Reactive programming tools using the WHATWG Streams API.

import { Accumulator } from '@johngw/stream-common/Function'
import { Flushable, pipeFlushes } from '@johngw/stream-common/Stream'

/**
 * Reduces chunks in to one accumulator. Once the stream is cancelled,
 * the accumulator will be queued.
 *
 * @group Transformers
 * @see {@link accumulate:function}
 * @example
 *
 * --a-------b-------c-----|
 *
 * reduce([], (acc, chunk) => [...acc, chunk])
 *
 * ------------------------[a,b,c]-|
 */
export function reduce<I, O>(
  acc: O,
  fn: Accumulator<I, O>,
  options?: Flushable
) {
  return new TransformStream<I, O>({
    start(controller) {
      pipeFlushes(
        () => controller.enqueue(acc),
        (error) => controller.error(error),
        options
      )
    },

    async transform(chunk, controller) {
      try {
        acc = await fn(acc, chunk)
      } catch (error) {
        controller.error(error)
      }
    },

    flush(controller) {
      controller.enqueue(acc)
    },
  })
}