Add type check for onclose callback (#80)

boot.js

@@ -221,6 +221,11 @@ Boot.prototype.after = function (func) {
 Boot.prototype.onClose = function (func) {
   // this is used to distinguish between onClose and close handlers
   // because they share the same queue but must be called with different signatures
+
+  if (typeof func !== 'function') {
+    throw new Error('not a function')
+  }
+
   func[this._isOnCloseHandlerKey] = true
   this._closeQ.unshift(func, callback.bind(this))
 

test/close.test.js

@@ -476,3 +476,14 @@ test('close with async onClose handlers', (t) => {
     })
   })
 })
+
+test('onClose callback must be a function', (t) => {
+  t.plan(1)
+
+  const app = boot()
+
+  app.use(function (server, opts, done) {
+    t.throws(() => app.onClose({}), { message: 'not a function' })
+    done()
+  })
+})