Advanced GraphQL - Custom Scalars

Overview of Custom Scalars

Custom scalars in GraphQL allow you to define your own data types that can be used in your schema. This is useful when you need to handle special data formats that are not covered by the default scalar types.

Creating a Custom Scalar

Defining the Scalar

To create a custom scalar, you need to define its behavior for serialization, parsing, and validation.

const { GraphQLScalarType } = require('graphql');

const CustomDate = new GraphQLScalarType({
  name: 'CustomDate',
  description: 'A custom date scalar type',
  serialize(value) {
    return value.getTime(); // Convert outgoing Date to timestamp
  },
  parseValue(value) {
    return new Date(value); // Convert incoming timestamp to Date
  },
  parseLiteral(ast) {
    if (ast.kind === Kind.INT) {
      return new Date(parseInt(ast.value, 10)); // Convert hard-coded AST string to Date
    }
    return null; // Invalid hard-coded value (not an integer)
  },
});
          

Using Custom Scalars in Your Schema

After defining your custom scalar, you can use it in your GraphQL schema just like any other type.

const { gql } = require('apollo-server');

const typeDefs = gql`
  scalar CustomDate

  type User {
    id: ID!
    name: String!
    birthdate: CustomDate
  }

  type Query {
    users: [User]
  }
`;
          

Best Practices for Custom Scalars

Follow these best practices when creating custom scalars:

• Keep It Simple: Ensure that the scalar is straightforward and serves a clear purpose.
• Validate Input: Implement validation logic to ensure that incoming data adheres to expected formats.
• Document Your Scalars: Provide clear documentation for the custom scalars to guide developers using them.
• Test Thoroughly: Write tests to ensure the custom scalar behaves as expected in various scenarios.

Summary

This guide provided an overview of creating and using custom scalar types in GraphQL. By implementing custom scalars, you can enhance the flexibility and robustness of your GraphQL schemas.