Home/Questions/Quick solution to fix the error: cannot return null for non-nullable field
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Bryce White
  • 25
Asked: 2022-05-18T21:09:35+00:00 In:

Quick solution to fix the error: cannot return null for non-nullable field

  • 25

I don’t know what I’m doing wrong, but I’ve already lost a couple of days struggling with this. Here is my command line:

export default `
 
 type User {
  id: ID!
  name: String!
  email: String!
 }
 
 type Query {
  allUsers: [User]
  currentUser: User
 }
 
 type Mutation {
  createAccount(name: String!, email: String!, password: String!): User
  loginUser(email: String!, password: String!): User
  updatePassword(email: String!, password: String!, newPassword: String!): User
  deleteAccount(email: String!, password: String!): User
 }
 
 `
createAccount: async (
  parent,
  { name, email, password },
  { User },
  info
 ) => {
  try {
  // Check for invalid (undefined) credentials
  if (!name || !email || !password) {
  return 'Please provide valid credentials';
  }
 
  // Check if there is a user with the same email
  const foundUser = await User.findOne({ email });
 
  if (foundUser) {
  return 'Email is already in use';
  }
 
  // If no user with email create a new user
  const hashedPassword = await bcrypt.hash(password, 10);
  await User.insert({ name, email, password: hashedPassword });
 
  const savedUser = await User.findOne({ email });
 
  return savedUser;
  } catch (error) {
  return error.message;
  }
 },

This returns:

Cannot return null for non-nullable field

I don’t have any experience with the “cannot return null for non-nullable field.” In this case, how should I change?

cannot return null for non-nullable field
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1 Answer

  1. Best Answer
    Expert

    The cause:

    The main issue with your resolver is that, in a variety of situations, it returns a string rather than a User object. According to your schema, createAccount can either return a User or null. Your resolver forces a returned string into an object because it was expecting one and then begins searching for User attributes on that object (such as email and name). As these are non-null properties on your User object, returning null or undefined for them causes an error because they don’t exist.

    Solution:

    Most likely, your resolver should just throw any errors that are required. They will then be included in the errors array that makes up your answer. For instance:

    // Check whether there is a user with the similar email or not
const foundUser = await User.findOne({ email })

if (foundUser) throw new Error('Email is already in use')

// If there is no user with email, let's create a new user
const hashedPassword = await bcrypt.hash(password, 10);
await User.insert({ name, email, password: hashedPassword });

const savedUser = await User.findOne({ email });

return savedUser;

    The response will look like the following if you receive a duplicate email:

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

    Use the formatResponse configuration parameters or formatError for your middleware of Apollo server if you wish to change how errors are displayed to clients. Using custom errors, which let you add specific properties like code to more readily identify the problem type on the client side, is also a smart idea.

    Finally, because your schema already has these inputs tagged as non-null, GraphQL will not need to check whether email, name or password are specified inside your resolver and will instead return an error if any of them are missing.

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