feat(header): add filtering by header field name function

Author: TomokiMiyauci

README.md

@@ -139,6 +139,28 @@ assertEquals(isSingletonField("Origin"), true);
 assertEquals(isSingletonField("Vary"), false);
 ```
 
+## filterKeys
+
+Returns a new {@link Headers} with all entries of the given headers except the
+ones that have a key(header name or field name) that does not match the given
+predicate.
+
+```ts
+import { filterKeys } from "https://deno.land/x/http_utils@$VERSION/mod.ts";
+import { assert } from "https://deno.land/std@$VERSION/testing/asserts.ts";
+
+const headers = filterKeys(
+  new Headers({
+    "date": "<date>",
+    "content-type": "<content-type>",
+  }),
+  (key) => key.startsWith("content"),
+);
+
+assert(headers.has("content-type"));
+assert(!headers.has("date"));
+```
+
 ## MessageMetadataHeader
 
 HTTP Message Metadata header fields.

header.ts

@@ -173,6 +173,32 @@ export function parseFieldValue(fieldValue: string): string[] {
     []).map((value) => value.trim()).filter(Boolean);
 }
 
+/** Returns a new {@link Headers} with all entries of the given headers except the ones that have a key(header name or field name) that does not match the given predicate.
+ *
+ * @example
+ * ```ts
+ * import { filterKeys } from "https://deno.land/x/http_utils@$VERSION/mod.ts";
+ * import { assert } from "https://deno.land/std@$VERSION/testing/asserts.ts";
+ *
+ * const headers = filterKeys(
+ *   new Headers({
+ *     "date": "<date>",
+ *     "content-type": "<content-type>",
+ *   }),
+ *   (key) => key.startsWith("content"),
+ * );
+ *
+ * assert(headers.has("content-type"));
+ * assert(!headers.has("date"));
+ * ```
+ */
+export function filterKeys(
+  headers: Headers,
+  predicate: (key: string) => boolean,
+): Headers {
+  return new Headers([...headers].filter(([key]) => predicate(key)));
+}
+
 /** HTTP Message Metadata header fields.
  * @see [RFC 9110, 6.6. Message Metadata](https://www.rfc-editor.org/rfc/rfc9110.html#section-6.6)
  *

header_test.ts

@@ -1,10 +1,11 @@
 import {
   equalsHeaders,
+  filterKeys,
   isSingletonField,
   mergeHeaders,
   parseFieldValue,
 } from "./header.ts";
-import { describe, expect, Fn, it } from "./dev_deps.ts";
+import { assert, describe, expect, Fn, it } from "./dev_deps.ts";
 
 describe("equalsHeaders", () => {
   it("should pass", () => {
@@ -212,3 +213,29 @@ Deno.test("parseFieldValue should pass", () => {
     expect(parseFieldValue(actual)).toEqual(expected);
   });
 });
+
+describe("filterKeys", () => {
+  it("should return new headers filtered by predicate", () => {
+    const table: [Headers, (key: string) => boolean, Headers][] = [
+      [
+        new Headers({ "x-test": "test", "date": "xxx" }),
+        () => true,
+        new Headers({ "x-test": "test", "date": "xxx" }),
+      ],
+      [
+        new Headers({ "x-test": "test", "date": "xxx" }),
+        () => false,
+        new Headers(),
+      ],
+      [
+        new Headers({ "x-test": "test", "date": "xxx" }),
+        (key) => key === "date",
+        new Headers({ date: "xxx" }),
+      ],
+    ];
+
+    table.forEach(([headers, predicate, expected]) => {
+      assert(equalsHeaders(filterKeys(headers, predicate), expected));
+    });
+  });
+});

mod.ts

@@ -8,6 +8,7 @@ export {
   ConditionalHeader,
   ContentNegotiationHeader,
   equalsHeaders,
+  filterKeys,
   type HttpFieldName,
   isSingletonField,
   mergeHeaders,