Scalar Type Definition

The leaf values of any request and input values to arguments are Scalars (or Enums) and are defined with a name and a series of functions used to parse input from ast or variables and to ensure validity.

If a type's serialize function returns null or does not return a value (i.e. it returns undefined) then an error will be raised and a null value will be returned in the response. It is always better to validate

Example:

const OddType = new GraphQLScalarType({
  name: 'Odd',
  serialize(value) {
    if (!Number.isFinite(value)) {
      throw new Error(
        `Scalar "Odd" cannot represent "${value}" since it is not a finite number.`,
      );
    }

    if (value % 2 === 0) {
      throw new Error(`Scalar "Odd" cannot represent "${value}" since it is even.`);
    }
    return value;
  }
});