GraphQL Scalars

Scalars are used to move parsing, validation and serialization of data types out from resolvers.

Scalars have 3 methods:

serialize(value) converts a object to a string, number or boolean.
This value comes from our resolvers and the result is sent to the client.

parseValue(value) converts a string, number or boolean to an object.
This value comes from the client and the result is passed to the resolver.

parseLiteral(valueNode, variables?) coverts AST ValueNode to an object.
The value is defined in the query and we are given the parsed AST representation containing:
{ kind: string, value: string }
The variables can be accessed here as well, if any are provided.

All of 3 these methods can return undefined (or not return anything) to indicate an invalid value. Or an Error can be thrown to show a custom error message.

Date Scalar

Date is probably the most used non-standard scalar and graphql-plugins comes with a built-in version that serializes Date objects as the number of milliseconds since 1 January, 1970 UTC.

This is also a great template for creating new custom scalars.

export const DateDef = `
scalar Date
`;

@Scalar('Date', DateDef)
export class DateScalar implements IScalar<Date, number>
{
    public serialize(value: Date): number
    {
        return value.valueOf();
    }

    public parseValue(value: number): Date | void
    {
        if (typeof value === 'number')
        {
            return new Date(value);
        }
    }

    public parseLiteral(valueNode: ValueNode, variables?: { [p:string]: any }): Date | void
    {
        if (valueNode.kind === Kind.INT)
        {
            const value = parseInt(valueNode.value);
            if (!isNaN(value))
            {
                return new Date(value);
            }
        }
    }
}

RegExp Scalar

Regular expressions are great for filtering data, so graphql-plugins comes with a built-in version of this scalar.

The serialized type is a string and it can use one of 2 formats:

• "/regexp?/g" version with slashes allows us to add modifiers
• "regexp?" version without modifiers