Add user defined proxy via middleware (#5073)

* Add user defined proxy via middleware

* Pass app, instead

packages/react-scripts/config/paths.js

@@ -58,6 +58,7 @@ module.exports = {
   appSrc: resolveApp('src'),
   yarnLockFile: resolveApp('yarn.lock'),
   testsSetup: resolveApp('src/setupTests.js'),
+  proxySetup: resolveApp('src/setupProxy.js'),
   appNodeModules: resolveApp('node_modules'),
   publicUrl: getPublicUrl(resolveApp('package.json')),
   servedPath: getServedPath(resolveApp('package.json')),
@@ -78,6 +79,7 @@ module.exports = {
   appSrc: resolveApp('src'),
   yarnLockFile: resolveApp('yarn.lock'),
   testsSetup: resolveApp('src/setupTests.js'),
+  proxySetup: resolveApp('src/setupProxy.js'),
   appNodeModules: resolveApp('node_modules'),
   publicUrl: getPublicUrl(resolveApp('package.json')),
   servedPath: getServedPath(resolveApp('package.json')),
@@ -108,6 +110,7 @@ if (
     appSrc: resolveOwn('template/src'),
     yarnLockFile: resolveOwn('template/yarn.lock'),
     testsSetup: resolveOwn('template/src/setupTests.js'),
+    proxySetup: resolveOwn('template/src/setupProxy.js'),
     appNodeModules: resolveOwn('node_modules'),
     publicUrl: getPublicUrl(resolveOwn('package.json')),
     servedPath: getServedPath(resolveOwn('package.json')),

packages/react-scripts/config/webpackDevServer.config.js

@@ -14,6 +14,7 @@ const noopServiceWorkerMiddleware = require('react-dev-utils/noopServiceWorkerMi
 const ignoredFiles = require('react-dev-utils/ignoredFiles');
 const config = require('./webpack.config.dev');
 const paths = require('./paths');
+const fs = require('fs');
 
 const protocol = process.env.HTTPS === 'true' ? 'https' : 'http';
 const host = process.env.HOST || '0.0.0.0';
@@ -81,7 +82,7 @@ module.exports = function(proxy, allowedHost) {
     },
     // Enable HTTPS if the HTTPS environment variable is set to 'true'
     https: protocol === 'https',
-    host: host,
+    host,
     overlay: false,
     historyApiFallback: {
       // Paths with dots should still use the history fallback.
@@ -91,11 +92,16 @@ module.exports = function(proxy, allowedHost) {
     public: allowedHost,
     proxy,
     before(app, server) {
+      if (fs.existsSync(paths.proxySetup)) {
+        // This registers user provided middleware for proxy reasons
+        require(paths.proxySetup)(app);
+      }
+
       // This lets us fetch source contents from webpack for the error overlay
       app.use(evalSourceMapMiddleware(server));
-
       // This lets us open files from the runtime error overlay.
       app.use(errorOverlayMiddleware());
+
       // This service worker file is effectively a 'no-op' that will reset any
       // previous service worker registered for the same host:port combination.
       // We do this in development to avoid hitting the production cache if