3 files changed, 31 insertions, 3 deletions
diff --git a/js/fetch.ts b/js/fetch.ts
index d66eb1c48..0a5f793a8 100644
--- a/js/fetch.ts
+++ b/js/fetch.ts
@@ -35,7 +35,7 @@ function hasHeaderValueOf(s: string, value: string): boolean {
 }
 
 class Body implements domTypes.Body, domTypes.ReadableStream, io.ReadCloser {
-  bodyUsed = false;
+  private _bodyUsed = false;
   private _bodyPromise: null | Promise<ArrayBuffer> = null;
   private _data: ArrayBuffer | null = null;
   readonly locked: boolean = false; // TODO
@@ -223,6 +223,7 @@ class Body implements domTypes.Body, domTypes.ReadableStream, io.ReadCloser {
   }
 
   read(p: Uint8Array): Promise<number | io.EOF> {
+    this._bodyUsed = true;
     return read(this.rid, p);
   }
 
@@ -245,6 +246,10 @@ class Body implements domTypes.Body, domTypes.ReadableStream, io.ReadCloser {
   [Symbol.asyncIterator](): AsyncIterableIterator<Uint8Array> {
     return io.toAsyncIterator(this);
   }
+
+  get bodyUsed(): boolean {
+    return this._bodyUsed;
+  }
 }
 
 export class Response implements domTypes.Response {
@@ -252,7 +257,6 @@ export class Response implements domTypes.Response {
   readonly redirected: boolean;
   headers: domTypes.Headers;
   readonly trailer: Promise<domTypes.Headers>;
-  bodyUsed = false;
   readonly body: Body;
 
   constructor(
@@ -302,6 +306,10 @@ export class Response implements domTypes.Response {
     return 200 <= this.status && this.status < 300;
   }
 
+  get bodyUsed(): boolean {
+    return this.body.bodyUsed;
+  }
+
   clone(): domTypes.Response {
     if (this.bodyUsed) {
       throw new TypeError(
diff --git a/js/fetch_test.ts b/js/fetch_test.ts
index 8d8b581d8..083d5333c 100644
--- a/js/fetch_test.ts
+++ b/js/fetch_test.ts
@@ -1,5 +1,11 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
-import { test, testPerm, assert, assertEquals } from "./test_util.ts";
+import {
+  test,
+  testPerm,
+  assert,
+  assertEquals,
+  assertThrows
+} from "./test_util.ts";
 
 testPerm({ net: true }, async function fetchJsonSuccess(): Promise<void> {
   const response = await fetch("http://localhost:4545/package.json");
@@ -38,6 +44,19 @@ testPerm({ net: true }, async function fetchBlob(): Promise<void> {
   assertEquals(blob.size, Number(headers.get("Content-Length")));
 });
 
+testPerm({ net: true }, async function fetchBodyUsed(): Promise<void> {
+  const response = await fetch("http://localhost:4545/package.json");
+  assertEquals(response.bodyUsed, false);
+  assertThrows(
+    (): void => {
+      // Assigning to read-only property throws in the strict mode.
+      response.bodyUsed = true;
+    }
+  );
+  await response.blob();
+  assertEquals(response.bodyUsed, true);
+});
+
 testPerm({ net: true }, async function fetchAsyncIterator(): Promise<void> {
   const response = await fetch("http://localhost:4545/package.json");
   const headers = response.headers;
diff --git a/js/test_util.ts b/js/test_util.ts
index bcbacf281..52e09301b 100644
--- a/js/test_util.ts
+++ b/js/test_util.ts
@@ -14,6 +14,7 @@ import {
 } from "./deps/https/deno.land/std/testing/asserts.ts";
 export {
   assert,
+  assertThrows,
   assertEquals,
   assertMatch,
   assertNotEquals,