Why am I getting a "Cannot return null for non-nullable field" error when doing a mutation? [duplicate]

The biggest problem with your resolver is that in any number of scenarios, instead of returning a User object, you return a string. Your schema specifies that createAccount can return a User or null (if it was User!, it would be non-nullable and then null would not be a valid type either).

When you return a string in your resolver, because it's expecting an object, it coerces it into one and then starts looking for User properties on that object (like name and email). Those properties don't exist, and because they are non-null properties on your User object, returning null/undefined for them results in an error.

Your resolver should probably just throw whatever errors it needs to throw. Then they will be returned as part of the errors array in your response. For example:

// Check if there is a user with the same emailconst foundUser = await User.findOne({ email })if (foundUser) throw new Error('Email is already in use')// If no user with email create a new userconst hashedPassword = await bcrypt.hash(password, 10);await User.insert({ name, email, password: hashedPassword });const savedUser = await User.findOne({ email });return savedUser;

Now, if you have a duplicate email, the response will look something like this:

{  "data": {    "createAccount": null  },  "errors": [    {      "message": "Email is already in use",      "locations": [        {          "line": 4,          "column": 3        }      ],      "path": [        "createAccount"      ]    }  ]}

If you want to manipulate how your errors are shown on the client, you should utilize the formatError or formatResponse configuration options for your Apollo server middleware. It's also good practice to use custom errors, allowing you to add custom properties like code to more easily identify the error type on the client side.

Lastly, it's unnecessary to check if name, email or password are defined inside your resolver -- your schema already has these inputs marked as non-null, which means GraphQL will automatically return an error if any of them are missing.