Improve error message when autoloading invalid view engine

fixes #3403

History.md

@@ -1,3 +1,8 @@
+unreleased
+==========
+
+  * Improve error message when autoloading invalid view engine
+
 4.15.5 / 2017-09-24
 ===================
 

lib/view.js

@@ -76,7 +76,15 @@ function View(name, options) {
     // load engine
     var mod = this.ext.substr(1)
     debug('require "%s"', mod)
-    opts.engines[this.ext] = require(mod).__express
+
+    // default engine export
+    var fn = require(mod).__express
+
+    if (typeof fn !== 'function') {
+      throw new Error('Module "' + mod + '" does not provide a view engine.')
+    }
+
+    opts.engines[this.ext] = fn
   }
 
   // store loaded engine

test/res.render.js

@@ -35,6 +35,20 @@ describe('res', function(){
       .expect('<p>tobi</p>', done);
     })
 
+    it('should error without "view engine" set and file extension to a non-engine module', function (done) {
+      var app = createApp()
+
+      app.locals.user = { name: 'tobi' }
+
+      app.use(function (req, res) {
+        res.render(path.join(__dirname, 'fixtures', 'broken.send'))
+      })
+
+      request(app)
+      .get('/')
+      .expect(500, /does not provide a view engine/, done)
+    })
+
     it('should error without "view engine" set and no file extension', function (done) {
       var app = createApp();