graphql, union scalar type?

Question

The payload field can be Int or String scalar type. when I write it like union type:

const schema = `
  input QuickReply {
    content_type: String
    title: String
    payload: Int | String
    image_url: String
  }
`

I got an error:

GraphQLError: Syntax Error GraphQL request (45:18) Expected Name, found |

    44:     title: String
    45:     payload: Int | String
                         ^
    46:     image_url: String

It seems GraphQL does not support union scalar type.

So, how can I solve this situation?

Answer 1

Scalars can't be used as part of unions, since per the specification, unions specifically "represent an object that could be one of a list of GraphQL Object types." Instead, you can use a custom scalar. For example:

const MAX_INT = 2147483647
const MIN_INT = -2147483648
const coerceIntString = (value) => {
  if (Array.isArray(value)) {
    throw new TypeError(`IntString cannot represent an array value: [${String(value)}]`)
  }
  if (Number.isInteger(value)) {
    if (value < MIN_INT || value > MAX_INT) {
      throw new TypeError(`Value is integer but outside of valid range for 32-bit signed integer: ${String(value)}`)
    }
    return value
  }
  return String(value)
}
const IntString = new GraphQLScalarType({
  name: 'IntString',
  serialize: coerceIntString,
  parseValue: coerceIntString,
  parseLiteral(ast) {
    if (ast.kind === Kind.INT) {
      return coerceIntString(parseInt(ast.value, 10))
    }
    if (ast.kind === Kind.STRING) {
      return ast.value
    }
    return undefined
  }
})

This code effectively combines the behaviors for both the Int and String types, while still enforcing the range for 32-bit signed integers. However, you could have whatever type coercion behavior you want. Check out the source code to see how the built-in scalars work, or this article for more details around how custom scalars work.