How to define JSON custom scalar type in graphql which can be nullable?

[Yifeng-Sigma]

#3532 (comment)
I want to define a JSON custom scalar type which can be nullable.

after graphql 16, serialize() can not return null, so I'm getting GRAPHQL error {"errors":[{"message":"Expected JsonType.serialize("null") to return non-nullable value, returned: null" error where JsonType is my customized scalar.

Wondering what's the recommended way to do this?

[MukundaKatta]

The v16 change requires serialize to return a non-null value. For a nullable JSON scalar, just return a sentinel on null input — you want the caller to receive null, which happens when the field resolver returns null, not when the scalar serializes one. So inside serialize, if you receive null throw (it should never be called with null if the field is nullable); the GraphQL executor skips serialize for null field values entirely.

[yudin-s]

Nullability should be modeled on the field type, not inside the scalar.

For example:

type Example {
  metadata: JSON
  requiredMetadata: JSON!
}

metadata: JSON means the field may be null. If the resolved field value is actually null, GraphQL should complete it as null before calling your scalar serialization logic.

So the scalar implementation should focus on valid JSON values when it is called:

const JSONScalar = new GraphQLScalarType({
  name: 'JSON',
  serialize(value) {
    if (value === null || value === undefined) {
      throw new TypeError('JSON scalar cannot serialize null directly')
    }
    return value
  },
  parseValue(value) {
    return value
  },
})

If serialize(null) is being called, I would check the resolver return path and the field type. Common causes are returning the string "null" from storage, wrapping the scalar in custom code before GraphQL completes null values, or marking the field as non-null somewhere in the schema.

In short: JSON can be nullable as a field, but the scalar itself does not need to “support null” as a JSON value.