Add helper to turn deno.Reader into async iterator (#1130)

js/deno.ts

@@ -7,6 +7,7 @@ export { chdir, cwd } from "./dir";
 export { File, open, stdin, stdout, stderr, read, write, close } from "./files";
 export {
   copy,
+  toAsyncIterator,
   ReadResult,
   Reader,
   Writer,

js/files_test.ts

@@ -17,3 +17,15 @@ test(async function filesCopyToStdout() {
   assertEqual(bytesWritten, fileSize);
   console.log("bytes written", bytesWritten);
 });
+
+test(async function filesToAsyncIterator() {
+  const filename = "tests/hello.txt";
+  const file = await deno.open(filename);
+
+  let totalSize = 0;
+  for await (const buf of deno.toAsyncIterator(file)) {
+    totalSize += buf.byteLength;
+  }
+
+  assertEqual(totalSize, 12);
+});

js/io.ts

@@ -115,3 +115,30 @@ export async function copy(dst: Writer, src: Reader): Promise<number> {
   }
   return n;
 }
+
+/**
+ * Turns `r` into async iterator.
+ *
+ *    for await (const chunk of readerIterator(reader)) {
+ *        console.log(chunk)
+ *    }
+ */
+export function toAsyncIterator(
+  r: Reader
+): AsyncIterableIterator<ArrayBufferView> {
+  const b = new Uint8Array(1024);
+
+  return {
+    [Symbol.asyncIterator]() {
+      return this;
+    },
+
+    async next(): Promise<IteratorResult<ArrayBufferView>> {
+      const result = await r.read(b);
+      return {
+        value: b.subarray(0, result.nread),
+        done: result.eof
+      };
+    }
+  };
+}