Add copyTo (#5)

README.md

@@ -107,6 +107,17 @@ export default async function handler(request: Request): Promise<Response> {
 }
 ```
 
+Some frameworks use a readonly `Response` object, so you need to use an existing `headers` object. In this case you can use the `copyTo` method to copy the headers to the response:
+
+```astro
+---
+import { CacheHeaders, ONE_HOUR } from "cdn-cache-control";
+
+new CacheHeaders().swr(ONE_HOUR).copyTo(Astro.response.headers);
+---
+
+```
+
 ## API
 
 <!-- TSDOC_START -->
@@ -168,6 +179,7 @@ Number of seconds in one year
 - [immutable](#gear-immutable)
 - [ttl](#gear-ttl)
 - [toObject](#gear-toobject)
+- [copyTo](#gear-copyto)
 - [getCdnCacheControl](#gear-getcdncachecontrol)
 - [setCdnCacheControl](#gear-setcdncachecontrol)
 - [getCacheControl](#gear-getcachecontrol)
@@ -233,6 +245,14 @@ Returns the headers as a plain object.
 | ---------- | ------------------------------ |
 | `toObject` | `() => Record<string, string>` |
 
+#### :gear: copyTo
+
+Copy the headers from this instance to another Headers instance.
+
+| Method   | Type                                   |
+| -------- | -------------------------------------- |
+| `copyTo` | `<T extends Headers>(headers: T) => T` |
+
 #### :gear: getCdnCacheControl
 
 The parsed cache-control header for the CDN cache.
@@ -284,3 +304,7 @@ The parsed content of the cache tags header.
 ```
 
 ```
+
+```
+
+```

index.test.js

@@ -105,4 +105,19 @@ describe("CacheHeaders", () => {
       "cache-tag": "tag1,tag2,tag3",
     });
   });
+
+  it("copies headers to an existing object", () => {
+    const existing = new Headers([["x-foo", "bar"]]);
+    const headers = new CacheHeaders().swr().tag("tag1").tag("tag2", "tag3");
+
+    const copied = headers.copyTo(existing);
+
+    assert.strictEqual(existing.get("x-foo"), "bar");
+    assert.strictEqual(
+      existing.get("CDN-Cache-Control"),
+      "public,s-maxage=0,stale-while-revalidate=604800",
+    );
+    assert.strictEqual(existing.get("Cache-Tag"), "tag1,tag2,tag3");
+    assert.strictEqual(existing, copied);
+  });
 });

src/index.ts

@@ -173,6 +173,17 @@ export class CacheHeaders extends Headers {
     return Object.fromEntries(this.entries());
   }
 
+  /**
+   * Copy the headers from this instance to another Headers instance.
+   */
+
+  copyTo<T extends Headers>(headers: T): T {
+    this.forEach((value, key) => {
+      headers.set(key, value);
+    });
+    return headers;
+  }
+
   private get cacheTagHeaderName(): string {
     switch (this.#cdn) {
       case "netlify":