fix(validation): Added support for middleware field validation

src/index.ts

@@ -82,6 +82,13 @@ function validateMiddleware(
             `Field ${type}.${field} exists in middleware but is missing in Schema.`,
           )
         }
+
+        if (!isMiddlewareFunction(middleware[type][field])) {
+          throw new MiddlewareError(
+            `Expected ${type}.${field} to be a function but found ` +
+              typeof middleware[type][field],
+          )
+        }
       })
     }
   })

test.js

@@ -121,7 +121,7 @@ const schemaMiddlewareAfter = async (resolve, parent, args, context, info) => {
   return 'changed'
 }
 
-// Wrong Middleware
+// Middleware Validation
 
 const middlewareWithUndefinedType = {
   Wrong: () => ({}),
@@ -133,6 +133,12 @@ const middlewareWithUndefinedField = {
   },
 }
 
+const middlewareWithObjectField = {
+  Query: {
+    before: false,
+  },
+}
+
 // Test ----------------------------------------------------------------------
 
 // Field
@@ -519,3 +525,16 @@ test('Middleware Error - Schema undefined field', async t => {
     ),
   )
 })
+
+test('Middleware Error - Middleware field is not a function.', async t => {
+  const schema = getSchema()
+
+  const res = t.throws(() => {
+    applyMiddleware(schema, middlewareWithObjectField)
+  })
+
+  t.deepEqual(
+    res,
+    MiddlewareError(`Expected Query.before to be a function but found boolean`),
+  )
+})