feat: allow headers to accept fn

README.md

@@ -70,12 +70,34 @@ This property allows a user to pass the list of HTTP request methods accepted by
 
 ### headers
 
-Type: `Object`  
+Type: `Object|Function`
 Default: `undefined`
 
 This property allows a user to pass custom HTTP headers on each request.
 eg. `{ "X-Custom-Header": "yes" }`
 
+or
+
+```js
+webpackDevMiddleware(compiler, {
+  headers: () => {
+    return {
+      'Last-Modified': new Date(),
+    };
+  },
+});
+```
+
+or
+
+```js
+webpackDevMiddleware(compiler, {
+  headers: (req, res, context) => {
+    res.setHeader('Last-Modified', new Date());
+  },
+});
+```
+
 ### index
 
 Type: `Boolean|String`  

src/middleware.js

@@ -42,7 +42,12 @@ export default function wrapper(context) {
 
     async function processRequest() {
       const filename = getFilenameFromUrl(context, req.url);
-      const { headers } = context.options;
+      let { headers } = context.options;
+
+      if (typeof headers === 'function') {
+        headers = headers(req, res, context);
+      }
+
       let content;
 
       if (!filename) {

src/options.json

@@ -25,7 +25,14 @@
       }
     },
     "headers": {
-      "type": "object"
+      "anyOf": [
+        {
+          "type": "object"
+        },
+        {
+          "instanceof": "Function"
+        }
+      ]
     },
     "publicPath": {
       "description": "The `publicPath` specifies the public URL address of the output files when referenced in a browser.",

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

@@ -1,9 +1,23 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`validation should throw an error on the "headers" option with "1" value 1`] = `
+"Invalid options object. Dev Middleware has been initialized using an options object that does not match the API schema.
+ - options.headers should be one of these:
+   object { … } | function
+   Details:
+    * options.headers should be an object:
+      object { … }
+    * options.headers should be an instance of function."
+`;
+
 exports[`validation should throw an error on the "headers" option with "true" value 1`] = `
 "Invalid options object. Dev Middleware has been initialized using an options object that does not match the API schema.
- - options.headers should be an object:
-   object { … }"
+ - options.headers should be one of these:
+   object { … } | function
+   Details:
+    * options.headers should be an object:
+      object { … }
+    * options.headers should be an instance of function."
 `;
 
 exports[`validation should throw an error on the "index" option with "{}" value 1`] = `

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

@@ -1,9 +1,23 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`validation should throw an error on the "headers" option with "1" value 1`] = `
+"Invalid options object. Dev Middleware has been initialized using an options object that does not match the API schema.
+ - options.headers should be one of these:
+   object { … } | function
+   Details:
+    * options.headers should be an object:
+      object { … }
+    * options.headers should be an instance of function."
+`;
+
 exports[`validation should throw an error on the "headers" option with "true" value 1`] = `
 "Invalid options object. Dev Middleware has been initialized using an options object that does not match the API schema.
- - options.headers should be an object:
-   object { … }"
+ - options.headers should be one of these:
+   object { … } | function
+   Details:
+    * options.headers should be an object:
+      object { … }
+    * options.headers should be an instance of function."
 `;
 
 exports[`validation should throw an error on the "index" option with "{}" value 1`] = `

test/validation-options.test.js

@@ -28,8 +28,8 @@ describe('validation', () => {
       failure: [{}, true],
     },
     headers: {
-      success: [{ 'X-Custom-Header': 'yes' }],
-      failure: [true],
+      success: [{ 'X-Custom-Header': 'yes' }, () => {}],
+      failure: [true, 1],
     },
     publicPath: {
       success: ['/foo', '', 'auto', () => '/public/path'],