Add ETag support

Author: mjackson

packages/headers/CHANGELOG.md

@@ -134,6 +134,7 @@ header.getPreferred(['gzip', 'deflate']); // 'gzip'
   - `connection`
   - `contentEncoding`
   - `contentLanguage`
+  - `etag`
   - `host`
   - `location`
   - `referer`

packages/headers/src/lib/super-headers.test.ts

@@ -232,6 +232,11 @@ describe('SuperHeaders', () => {
       assert.equal(headers.get('Date'), 'Fri, 01 Jan 2021 00:00:00 GMT');
     });
 
+    it('handles the etag property', () => {
+      let headers = new SuperHeaders({ etag: 'abc' });
+      assert.equal(headers.get('ETag'), '"abc"');
+    });
+
     it('handles the expires property', () => {
       let headers = new SuperHeaders({ expires: new Date('2021-01-01T00:00:00Z') });
       assert.equal(headers.get('Expires'), 'Fri, 01 Jan 2021 00:00:00 GMT');
@@ -526,6 +531,30 @@ describe('SuperHeaders', () => {
       assert.equal(headers.date, null);
     });
 
+    it('supports the etag property', () => {
+      let headers = new SuperHeaders();
+
+      assert.equal(headers.etag, null);
+
+      headers.etag = 'abc';
+      assert.equal(headers.etag, '"abc"');
+
+      headers.etag = '"def"';
+      assert.equal(headers.etag, '"def"');
+
+      headers.etag = 'W/"def"';
+      assert.equal(headers.etag, 'W/"def"');
+
+      headers.etag = '';
+      assert.equal(headers.etag, '""');
+
+      headers.etag = '""';
+      assert.equal(headers.etag, '""');
+
+      headers.etag = null;
+      assert.equal(headers.etag, null);
+    });
+
     it('supports the expires property', () => {
       let headers = new SuperHeaders();
 

packages/headers/src/lib/super-headers.ts

@@ -72,6 +72,10 @@ interface SuperHeadersPropertyInit {
    * The [`Date`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Date) header value.
    */
   date?: string | DateInit;
+  /**
+   * The [`ETag`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag) header value.
+   */
+  etag?: string;
   /**
    * The [`Expires`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Expires) header value.
    */
@@ -543,6 +547,24 @@ export class SuperHeaders extends Headers {
     this.#setDateValue('date', value);
   }
 
+  /**
+   * The `ETag` header provides a unique identifier for the current version of the resource.
+   *
+   * [MDN `ETag` Reference](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag)
+   *
+   * [HTTP/1.1 Specification](https://datatracker.ietf.org/doc/html/rfc7232#section-2.3)
+   */
+  get etag(): string | null {
+    return this.get('etag');
+  }
+
+  set etag(value: string | undefined | null) {
+    this.#setValue(
+      'etag',
+      typeof value === 'string' && !/^(W\/)?".*"$/.test(value) ? `"${value}"` : value,
+    );
+  }
+
   /**
    * The `Expires` header contains the date/time after which the response is considered stale.
    *