feat: support fn in headers option

lib/Server.js

@@ -851,10 +851,14 @@ class Server {
   }
 
   setContentHeaders(req, res, next) {
-    if (this.options.headers) {
+    let { headers } = this.options;
+    if (headers) {
+      if (typeof headers === 'function') {
+        headers = headers(req, res, this.middleware.context);
+      }
       // eslint-disable-next-line guard-for-in
-      for (const name in this.options.headers) {
-        res.setHeader(name, this.options.headers[name]);
+      for (const name in headers) {
+        res.setHeader(name, headers[name]);
       }
     }
 

lib/options.json

@@ -258,7 +258,14 @@
       "description": "Defines routes which are enabled by default, on by default and allows localhost. https://webpack.js.org/configuration/dev-server/#devserverfirewall"
     },
     "headers": {
-      "type": "object",
+      "anyOf": [
+        {
+          "type": "object"
+        },
+        {
+          "instanceof": "Function"
+        }
+      ],
       "description": "Adds headers to all responses. https://webpack.js.org/configuration/dev-server/#devserverheaders"
     },
     "historyApiFallback": {

test/__snapshots__/validate-options.test.js.snap.webpack4

@@ -122,11 +122,26 @@ exports[`options validate should throw an error on the "firewall" option with '[
  - configuration.firewall should be an non-empty array."
 `;
 
+exports[`options validate should throw an error on the "headers" option with '1' value 1`] = `
+"ValidationError: Invalid configuration object. Object has been initialized using a configuration object that does not match the API schema.
+ - configuration.headers should be one of these:
+   object { … } | function
+   -> Adds headers to all responses. https://webpack.js.org/configuration/dev-server/#devserverheaders
+   Details:
+    * configuration.headers should be an object:
+      object { … }
+    * configuration.headers should be an instance of function."
+`;
+
 exports[`options validate should throw an error on the "headers" option with 'false' value 1`] = `
 "ValidationError: Invalid configuration object. Object has been initialized using a configuration object that does not match the API schema.
- - configuration.headers should be an object:
-   object { … }
-   -> Adds headers to all responses. https://webpack.js.org/configuration/dev-server/#devserverheaders"
+ - configuration.headers should be one of these:
+   object { … } | function
+   -> Adds headers to all responses. https://webpack.js.org/configuration/dev-server/#devserverheaders
+   Details:
+    * configuration.headers should be an object:
+      object { … }
+    * configuration.headers should be an instance of function."
 `;
 
 exports[`options validate should throw an error on the "historyApiFallback" option with '' value 1`] = `

test/__snapshots__/validate-options.test.js.snap.webpack5

@@ -122,11 +122,26 @@ exports[`options validate should throw an error on the "firewall" option with '[
  - configuration.firewall should be an non-empty array."
 `;
 
+exports[`options validate should throw an error on the "headers" option with '1' value 1`] = `
+"ValidationError: Invalid configuration object. Object has been initialized using a configuration object that does not match the API schema.
+ - configuration.headers should be one of these:
+   object { … } | function
+   -> Adds headers to all responses. https://webpack.js.org/configuration/dev-server/#devserverheaders
+   Details:
+    * configuration.headers should be an object:
+      object { … }
+    * configuration.headers should be an instance of function."
+`;
+
 exports[`options validate should throw an error on the "headers" option with 'false' value 1`] = `
 "ValidationError: Invalid configuration object. Object has been initialized using a configuration object that does not match the API schema.
- - configuration.headers should be an object:
-   object { … }
-   -> Adds headers to all responses. https://webpack.js.org/configuration/dev-server/#devserverheaders"
+ - configuration.headers should be one of these:
+   object { … } | function
+   -> Adds headers to all responses. https://webpack.js.org/configuration/dev-server/#devserverheaders
+   Details:
+    * configuration.headers should be an object:
+      object { … }
+    * configuration.headers should be an instance of function."
 `;
 
 exports[`options validate should throw an error on the "historyApiFallback" option with '' value 1`] = `

test/server/headers-option.test.js

@@ -55,6 +55,32 @@ describe('headers option', () => {
     });
   });
 
+  describe('as a function', () => {
+    beforeAll((done) => {
+      server = testServer.start(
+        config,
+        {
+          headers: () => {return { 'X-Bar': ['key1=value1', 'key2=value2'] }},
+          port,
+        },
+        done
+      );
+      req = request(server.app);
+    });
+
+    afterAll(testServer.close);
+
+    it('GET request with headers as a function', (done) => {
+      // https://github.com/webpack/webpack-dev-server/pull/1650#discussion_r254217027
+      const expected = ['v7', 'v8', 'v9'].includes(
+        process.version.split('.')[0]
+      )
+        ? 'key1=value1,key2=value2'
+        : 'key1=value1, key2=value2';
+      req.get('/main').expect('X-Bar', expected).expect(200, done);
+    });
+  });
+
   describe('dev middleware headers take precedence for dev middleware output files', () => {
     beforeAll((done) => {
       server = testServer.start(

test/validate-options.test.js

@@ -156,8 +156,8 @@ const tests = {
     failure: ['', []],
   },
   headers: {
-    success: [{}, { foo: 'bar' }],
-    failure: [false],
+    success: [{}, { foo: 'bar' }, () => {}],
+    failure: [false, 1],
   },
   historyApiFallback: {
     success: [{}, true],