Update extendBuild modules API

lib/common/utils.js

@@ -72,12 +72,20 @@ export function chainFn (base, fn) {
   if (!(fn instanceof Function)) {
     return
   }
-  return function () {
-    if (base instanceof Function) {
-      base.apply(this, arguments)
+  if (typeof base !== 'function') {
+    return fn.apply(this, arguments)
   }
-    fn.apply(this, arguments)
+  let baseResult = base.apply(this, arguments)
+  // Allow function to mutate the first argument instead of returning the result
+  if (baseResult === undefined) {
+    baseResult = arguments[0]
   }
+  let fnResult = fn.call(this, baseResult, ...arguments.slice(1))
+  // Return mutated argument if no result was returned
+  if (fnResult === undefined) {
+    return baseResult
+  }
+  return fnResult
 }
 
 export function isPureObject (o) {

lib/core/module.js

@@ -74,25 +74,7 @@ export default class ModuleContainer extends Tapable {
   }
 
   extendRoutes (fn) {
-    const extendRoutesFromConfig = this.options.router.extendRoutes
-    this.options.router.extendRoutes = function (routes, resolve) {
-      /* istanbul ignore if */
-      if (!(fn instanceof Function)) {
-        return
-      }
-      if (typeof extendRoutesFromConfig !== 'function') {
-        return fn.call(this, routes, resolve)
-      }
-      let extendedRoutes = extendRoutesFromConfig.call(this, routes, resolve)
-      if (extendedRoutes === undefined) {
-        extendedRoutes = routes
-      }
-      let moduleExtendedRoutes = fn.call(this, extendedRoutes, resolve)
-      if (moduleExtendedRoutes === undefined) {
-        return extendedRoutes
-      }
-      return moduleExtendedRoutes
-    }
+    this.options.router.extendRoutes = chainFn(this.options.router.extendRoutes, fn)
   }
 
   requireModule (moduleOpts) {

test/fixtures/module/modules/basic/index.js

@@ -15,6 +15,7 @@ module.exports = function basicModule (options, resolve) {
   // Extend build again
   this.extendBuild((config, { isClient, isServer }) => {
     // Do nothing!
+    return config
   })
 
   // Extend routes