$lastName of type String used in position expecting type String

Question

this may code

Schema

import { gql } from 'apollo-server-express';

export default gql`
  extend type Mutation {
    signUp(
      lastName: String!
    ): String!
  }
`;

Resolvers

{
  Query: {},
  Mutation: {
    signUp: async (
      _,
      { lastName}
    ) => {
      try {
        console.log(lastName)
        return 'ok';    
      } catch (error) {
        return 'error';
      }
    },
  },
};

Request

mutation($lastName:String){
  signUp(lastName:$lastName)
}

Query Veriables

{"lastName":"Darjo" }

I can’t understand, but I get Error

"Variable \"$lastName\" of type \"String\" used in position expecting type \"String!\".",

but when I remove the sign ! lastName: String everything is working.

I just can’t understand. What is the reason ?.

score 13 · Accepted Answer · answered Nov 18 '19 at 19:53

You've specified that the type of the lastName argument as String!. The ! indicates the type is non-null -- in other words, the argument is required and cannot equal null.

In your query, you're defining a variable ($lastName) and assigning it a type. An argument can be passed a variable, but only if their types match. For example, if the argument takes an Int, you cannot pass it a variable of the type Boolean. Similarly, if an argument is non-null, you cannot pass it a variable that is nullable. That's because a nullable variable could be null, but that would violate the non-nullability of the argument.

Note that the opposite is not true -- a nullable argument can accept a non-null variable.

This is valid:

# String! argument and String! variable
type Mutation {
  signUp(lastName: String!): String!
}

mutation($lastName:String!) {
  signUp(lastName:$lastName)
}

Also valid:

# String argument and String! variable
type Mutation {
  signUp(lastName: String): String!
}

mutation($lastName: String!) {
  signUp(lastName: $lastName)
}

As is this:

# String argument and String variable
type Mutation {
  signUp(lastName: String): String!
}

mutation($lastName: String) {
  signUp(lastName: $lastName)
}

But this is not valid:

# String! argument and String variable
type Mutation {
  signUp(lastName: String!): String!
}

mutation($lastName: String) {
  signUp(lastName: $lastName)
}

The only exception to this is if you provide a default value for the variable. So this is still valid:

# String! argument and String variable
type Mutation {
  signUp(lastName: String!): String!
}

mutation($lastName: String = "Some default value") {
  signUp(lastName: $lastName)
}

You can read about variable validation in the spec.

Thanks. In our case I know that all consumers are passing a non-null value, but I'm still not able to refactor our schema to make the String variable required. That seems a shame. — Justin Caldicott, Apr 23 '21 at 09:39

$lastName of type String used in position expecting type String

1 Answers1