Function transform

web.js:59–100

(transformFn, flushFn)

 * @returns {{ readable: ReadableStream<any>, writable: WritableStream<any> }}
 */
export function transform (transformFn, flushFn) {
  if (typeof transformFn !== 'function') {
    /** @type {ClassicWebTransformFn} */
    const passthrough = (chunk, controller) => controller.enqueue(chunk)
    transformFn = passthrough
  }

  const tKind = fnKind(transformFn)

  if (tKind === 'asyncgen') {
    return asyncGenTransform(/** @type {AsyncGenWebTransformFn} */ (transformFn), flushFn)
  }

  /** @type {Transformer<any, any>} */
  const inner = {}

  if (tKind === 'async') {
    const fn = /** @type {AsyncWebTransformFn} */ (transformFn)
    inner.transform = async (chunk, controller) => {
      const out = await fn(chunk)
      if (out !== undefined) controller.enqueue(out)
    }
  } else {
    const fn = /** @type {ClassicWebTransformFn} */ (transformFn)
    inner.transform = (chunk, controller) => fn(chunk, controller)
  }

  if (flushFn) {
    if (fnKind(flushFn) === 'async') {
      const fn = /** @type {AsyncWebFlushFn} */ (flushFn)
      inner.flush = async (controller) => {
        const out = await fn()
        if (out !== undefined) controller.enqueue(out)
      }
    } else {
      const fn = /** @type {ClassicWebFlushFn} */ (flushFn)
      inner.flush = (controller) => fn(controller)
    }
  }

  return new TransformStream(inner)
}

/**
 * Drive an async-generator transform as a `{ readable, writable }` pair.