2 files changed, 39 insertions, 0 deletions
diff --git a/cli/js/headers.ts b/cli/js/headers.ts
index dc0de54dd..cc22077ad 100644
--- a/cli/js/headers.ts
+++ b/cli/js/headers.ts
@@ -2,6 +2,7 @@
 import * as domTypes from "./dom_types.ts";
 import { DomIterableMixin } from "./mixins/dom_iterable.ts";
 import { requiredArguments } from "./util.ts";
+import { customInspect } from "./console.ts";
 
 // From node-fetch
 // Copyright (c) 2016 David Frank. MIT License.
@@ -85,6 +86,18 @@ class HeadersBase {
     }
   }
 
+  [customInspect](): string {
+    let headerSize = this[headerMap].size;
+    let output = "";
+    this[headerMap].forEach((value, key) => {
+      const prefix = headerSize === this[headerMap].size ? " " : "";
+      const postfix = headerSize === 1 ? " " : ", ";
+      output = output + `${prefix}${key}: ${value}${postfix}`;
+      headerSize--;
+    });
+    return `Headers {${output}}`;
+  }
+
   // ref: https://fetch.spec.whatwg.org/#concept-headers-append
   append(name: string, value: string): void {
     requiredArguments("Headers.append", arguments.length, 2);
diff --git a/cli/js/headers_test.ts b/cli/js/headers_test.ts
index f08283c51..a01abb6a6 100644
--- a/cli/js/headers_test.ts
+++ b/cli/js/headers_test.ts
@@ -1,5 +1,9 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 import { test, assert, assertEquals } from "./test_util.ts";
+const {
+  stringifyArgs
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+} = Deno as any;
 
 // Logic heavily copied from web-platform-tests, make
 // sure pass mostly header basic test
@@ -329,3 +333,25 @@ test(function toStringShouldBeWebCompatibility(): void {
   const headers = new Headers();
   assertEquals(headers.toString(), "[object Headers]");
 });
+
+function stringify(...args: unknown[]): string {
+  return stringifyArgs(args).replace(/\n$/, "");
+}
+
+test(function customInspectReturnsCorrectHeadersFormat(): void {
+  const blankHeaders = new Headers();
+  assertEquals(stringify(blankHeaders), "Headers {}");
+  const singleHeader = new Headers([["Content-Type", "application/json"]]);
+  assertEquals(
+    stringify(singleHeader),
+    "Headers { content-type: application/json }"
+  );
+  const multiParamHeader = new Headers([
+    ["Content-Type", "application/json"],
+    ["Content-Length", "1337"]
+  ]);
+  assertEquals(
+    stringify(multiParamHeader),
+    "Headers { content-type: application/json, content-length: 1337 }"
+  );
+});