Handle ETag and If-None-Match (#39)

* Automatically handle ETags

* Document what is happening

* Clarify with comment

* Better check, as range headers can be 0

* Clarify even more

README.md

@@ -182,6 +182,8 @@ Allows you to set custom headers (and overwrite the default ones) for certain pa
 }
 ```
 
+If you define the `ETag` header for a path, the handler will automatically reply with status code `304` for that path if a request comes in with a matching `If-None-Match` header.
+
 **NOTE:** The paths can only contain globs that are matched using [minimatch](https://github.com/isaacs/minimatch).
 
 ### directoryListing (Boolean|Array)

src/index.js

@@ -640,6 +640,19 @@ module.exports = async (request, response, config = {}, methods = {}) => {
 	const stream = await handlers.createReadStream(absolutePath);
 	const headers = await getHeaders(config.headers, current, absolutePath, stats);
 
+	// We need to check for `headers.ETag` being truthy first, otherwise it will
+	// match `undefined` being equal to `undefined`, which is true.
+	//
+	// Checking for `undefined` and `null` is also important, because `Range` can be `0`.
+	//
+	// eslint-disable-next-line no-eq-null
+	if (request.headers.range == null && headers.ETag && headers.ETag === request.headers['if-none-match']) {
+		response.statusCode = 304;
+		response.end();
+
+		return;
+	}
+
 	response.writeHead(response.statusCode || 200, headers);
 	stream.pipe(response);
 };

test/integration.js

@@ -1031,3 +1031,41 @@ test('modify config in `createReadStream` handler', async t => {
 	t.deepEqual(output, header.value);
 });
 
+test('automatically handle ETag headers for normal files', async t => {
+	const name = 'object.json';
+	const related = path.join(fixturesFull, name);
+	const content = await fs.readJSON(related);
+	const value = 'd2ijdjoi29f3h3232';
+
+	const url = await getUrl({
+		headers: [{
+			source: '**',
+			headers: [{
+				key: 'ETag',
+				value
+			}]
+		}]
+	});
+
+	const response = await fetch(`${url}/${name}`);
+	const {headers} = response;
+
+	const type = headers.get('content-type');
+	const eTag = headers.get('etag');
+
+	t.is(type, 'application/json; charset=utf-8');
+	t.is(eTag, value);
+
+	const text = await response.text();
+	const spec = JSON.parse(text);
+
+	t.deepEqual(spec, content);
+
+	const cacheResponse = await fetch(`${url}/${name}`, {
+		headers: {
+			'if-none-match': value
+		}
+	});
+
+	t.is(cacheResponse.status, 304);
+});