Add optional queuingStrategy parameter to transform

Author: twiss

lib/streams.js

@@ -222,12 +222,19 @@ function transformWithCancel(customCancel) {
 
 /**
  * Transform a stream using helper functions which are called on each chunk, and on stream close, respectively.
+ * Takes an optional queuing strategy for the resulting readable stream;
+ * see https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream/ReadableStream#queuingstrategy.
+ * By default, the queueing strategy is non-buffering. When the `process`
+ * function is asynchronous, it may be useful to pass a buffering
+ * queuing strategy to enable multiple chunks to be processed in parallel;
+ * e.g. pass `{ highWaterMark: 4 }` to process up to 4 chunks in parallel.
  * @param {ReadableStream|Uint8array|String} input
  * @param {Function} process
  * @param {Function} finish
+ * @param {Object} queuingStrategy
  * @returns {ReadableStream|Uint8array|String}
  */
-function transform(input, process = () => undefined, finish = () => undefined) {
+function transform(input, process = () => undefined, finish = () => undefined, queuingStrategy = { highWaterMark: 0 }) {
   if (isArrayStream(input)) {
     const output = new ArrayStream();
     (async () => {
@@ -288,7 +295,7 @@ function transform(input, process = () => undefined, finish = () => undefined) {
       async cancel(reason) {
         await reader.cancel(reason);
       }
-    }, { highWaterMark: 0 });
+    }, queuingStrategy);
   }
   const result1 = process(input);
   const result2 = finish();