Validate that given register is a function

Author: nhunzaker

src/__tests__/plugin-test.js

@@ -76,4 +76,21 @@ describe('Plugins', function() {
       instances[1].should.not.have.property('dirty')
     }, done)
   })
+
+  it ('throws an error if a register property is not a function', function(done) {
+    let app  = new Microcosm()
+
+    let Plugin = {
+      register: 'booyah'
+    }
+
+    app.addPlugin(Plugin)
+
+    try {
+      app.start()
+    } catch(error) {
+      expect(error).to.be.instanceOf(TypeError)
+      done()
+    }
+  })
 })

src/plugin.js

@@ -7,11 +7,19 @@
  * of a Microcosm.
  */
 
-function PluginFactory(config, options, app) {
+function PluginFactory (config, options, app) {
   return Object.assign({ app, options }, config)
 }
 
+function checkPlugin (plugin) {
+  if (process.env.NODE_ENV === 'development' && 'register' in plugin && typeof plugin.register !== 'function') {
+    throw TypeError('Expected register property of plugin to be a function, instead got ' + plugin.register)
+  }
+}
+
 function installPlugin (next, plugin) {
+  checkPlugin(plugin)
+
   return function (error) {
     // Halt execution of all future plugin installation if there is an error
     if (error) {