5 files changed, 169 insertions, 28 deletions
diff --git a/cli/js/deno.ts b/cli/js/deno.ts
index e35e8c161..b75df4a9d 100644
--- a/cli/js/deno.ts
+++ b/cli/js/deno.ts
@@ -42,7 +42,8 @@ export { FsEvent, fsEvents } from "./ops/fs_events.ts";
 export {
   EOF,
   copy,
-  toAsyncIterator,
+  iter,
+  iterSync,
   SeekMode,
   Reader,
   SyncReader,
diff --git a/cli/js/io.ts b/cli/js/io.ts
index b5af34224..023718100 100644
--- a/cli/js/io.ts
+++ b/cli/js/io.ts
@@ -84,23 +84,32 @@ export async function copy(dst: Writer, src: Reader): Promise<number> {
   return n;
 }
 
-export function toAsyncIterator(r: Reader): AsyncIterableIterator<Uint8Array> {
-  const b = new Uint8Array(1024);
-  return {
-    [Symbol.asyncIterator](): AsyncIterableIterator<Uint8Array> {
-      return this;
-    },
-
-    async next(): Promise<IteratorResult<Uint8Array>> {
-      const result = await r.read(b);
-      if (result === EOF) {
-        return { value: new Uint8Array(), done: true };
-      }
-
-      return {
-        value: b.subarray(0, result),
-        done: false,
-      };
-    },
-  };
+export async function* iter(
+  r: Reader,
+  bufSize?: number
+): AsyncIterableIterator<Uint8Array> {
+  const b = new Uint8Array(bufSize ?? 1024);
+  while (true) {
+    const result = await r.read(b);
+    if (result === EOF) {
+      break;
+    }
+
+    yield b.subarray(0, result);
+  }
+}
+
+export function* iterSync(
+  r: SyncReader,
+  bufSize?: number
+): IterableIterator<Uint8Array> {
+  const b = new Uint8Array(bufSize ?? 1024);
+  while (true) {
+    const result = r.readSync(b);
+    if (result === EOF) {
+      break;
+    }
+
+    yield b.subarray(0, result);
+  }
 }
diff --git a/cli/js/lib.deno.ns.d.ts b/cli/js/lib.deno.ns.d.ts
index 9f2c854d9..ed2e40f84 100644
--- a/cli/js/lib.deno.ns.d.ts
+++ b/cli/js/lib.deno.ns.d.ts
@@ -573,13 +573,61 @@ declare namespace Deno {
    */
   export function copy(dst: Writer, src: Reader): Promise<number>;
 
-  /** Turns a Reader, `r`, into an async iterator.
+  /** **UNSTABLE**: new API, yet to be vetted
+   * Turns a Reader, `r`, into an async iterator.
+   *
+   *      let f = await open("/etc/passwd");
+   *      for await (const chunk of iter(f)) {
+   *        console.log(chunk);
+   *      }
+   *      f.close();
+   *
+   * Second argument can be used to tune size of a buffer.
+   * Default size of the buffer is 1024 bytes.
    *
-   *      for await (const chunk of toAsyncIterator(reader)) {
+   *      let f = await open("/etc/passwd");
+   *      for await (const chunk of iter(f, 1024 * 1024)) {
    *        console.log(chunk);
    *      }
+   *      f.close();
+   *
+   * Iterator uses internal buffer of fixed size for efficiency returning
+   * a view on that buffer on each iteration. It it therefore callers
+   * responsibility to copy contents of the buffer if needed; otherwise
+   * next iteration will overwrite contents of previously returned chunk.
+   */
+  export function iter(
+    r: Reader,
+    bufSize?: number
+  ): AsyncIterableIterator<Uint8Array>;
+
+  /** **UNSTABLE**: new API, yet to be vetted
+   * Turns a SyncReader, `r`, into an iterator.
+   *
+   *      let f = await open("/etc/passwd");
+   *      for (const chunk of iterSync(reader)) {
+   *        console.log(chunk);
+   *      }
+   *      f.close();
+   *
+   * Second argument can be used to tune size of a buffer.
+   * Default size of the buffer is 1024 bytes.
+   *
+   *      let f = await open("/etc/passwd");
+   *      for (const chunk of iterSync(reader, 1024 * 1024)) {
+   *        console.log(chunk);
+   *      }
+   *      f.close()
+   *
+   * Iterator uses internal buffer of fixed size for efficiency returning
+   * a view on that buffer on each iteration. It it therefore callers
+   * responsibility to copy contents of the buffer if needed; otherwise
+   * next iteration will overwrite contents of previously returned chunk.
    */
-  export function toAsyncIterator(r: Reader): AsyncIterableIterator<Uint8Array>;
+  export function iterSync(
+    r: SyncReader,
+    bufSize?: number
+  ): IterableIterator<Uint8Array>;
 
   /** Synchronously open a file and return an instance of `Deno.File`.  The
    * file does not need to previously exist if using the `create` or `createNew`
diff --git a/cli/js/tests/files_test.ts b/cli/js/tests/files_test.ts
index 1f7352afc..f68e5def4 100644
--- a/cli/js/tests/files_test.ts
+++ b/cli/js/tests/files_test.ts
@@ -25,23 +25,71 @@ unitTest({ perms: { read: true } }, async function filesCopyToStdout(): Promise<
   file.close();
 });
 
+unitTest({ perms: { read: true } }, async function filesIter(): Promise<void> {
+  const filename = "cli/tests/hello.txt";
+  const file = await Deno.open(filename);
+
+  let totalSize = 0;
+  for await (const buf of Deno.iter(file)) {
+    totalSize += buf.byteLength;
+  }
+
+  assertEquals(totalSize, 12);
+  file.close();
+});
+
 unitTest(
   { perms: { read: true } },
-  async function filesToAsyncIterator(): Promise<void> {
+  async function filesIterCustomBufSize(): Promise<void> {
     const filename = "cli/tests/hello.txt";
     const file = await Deno.open(filename);
 
     let totalSize = 0;
-    for await (const buf of Deno.toAsyncIterator(file)) {
+    let iterations = 0;
+    for await (const buf of Deno.iter(file, 6)) {
+      totalSize += buf.byteLength;
+      iterations += 1;
+    }
+
+    assertEquals(totalSize, 12);
+    assertEquals(iterations, 2);
+    file.close();
+  }
+);
+
+unitTest({ perms: { read: true } }, function filesIterSync(): void {
+  const filename = "cli/tests/hello.txt";
+  const file = Deno.openSync(filename);
+
+  let totalSize = 0;
+  for (const buf of Deno.iterSync(file)) {
+    totalSize += buf.byteLength;
+  }
+
+  assertEquals(totalSize, 12);
+  file.close();
+});
+
+unitTest(
+  { perms: { read: true } },
+  function filesIterSyncCustomBufSize(): void {
+    const filename = "cli/tests/hello.txt";
+    const file = Deno.openSync(filename);
+
+    let totalSize = 0;
+    let iterations = 0;
+    for (const buf of Deno.iterSync(file, 6)) {
       totalSize += buf.byteLength;
+      iterations += 1;
     }
 
     assertEquals(totalSize, 12);
+    assertEquals(iterations, 2);
     file.close();
   }
 );
 
-unitTest(async function readerToAsyncIterator(): Promise<void> {
+unitTest(async function readerIter(): Promise<void> {
   // ref: https://github.com/denoland/deno/issues/2330
   const encoder = new TextEncoder();
 
@@ -69,7 +117,42 @@ unitTest(async function readerToAsyncIterator(): Promise<void> {
   const reader = new TestReader("hello world!");
 
   let totalSize = 0;
-  for await (const buf of Deno.toAsyncIterator(reader)) {
+  for await (const buf of Deno.iter(reader)) {
+    totalSize += buf.byteLength;
+  }
+
+  assertEquals(totalSize, 12);
+});
+
+unitTest(async function readerIterSync(): Promise<void> {
+  // ref: https://github.com/denoland/deno/issues/2330
+  const encoder = new TextEncoder();
+
+  class TestReader implements Deno.SyncReader {
+    #offset = 0;
+    #buf: Uint8Array;
+
+    constructor(s: string) {
+      this.#buf = new Uint8Array(encoder.encode(s));
+    }
+
+    readSync(p: Uint8Array): number | Deno.EOF {
+      const n = Math.min(p.byteLength, this.#buf.byteLength - this.#offset);
+      p.set(this.#buf.slice(this.#offset, this.#offset + n));
+      this.#offset += n;
+
+      if (n === 0) {
+        return Deno.EOF;
+      }
+
+      return n;
+    }
+  }
+
+  const reader = new TestReader("hello world!");
+
+  let totalSize = 0;
+  for await (const buf of Deno.iterSync(reader)) {
     totalSize += buf.byteLength;
   }
 
diff --git a/cli/js/web/fetch.ts b/cli/js/web/fetch.ts
index 2c16d5fb0..abb5f2aa2 100644
--- a/cli/js/web/fetch.ts
+++ b/cli/js/web/fetch.ts
@@ -248,7 +248,7 @@ class Body implements domTypes.Body, ReadableStream<Uint8Array>, io.ReadCloser {
   }
 
   [Symbol.asyncIterator](): AsyncIterableIterator<Uint8Array> {
-    return io.toAsyncIterator(this);
+    return io.iter(this);
   }
 
   get bodyUsed(): boolean {