Creating Custom GraphQL Scalars for Advanced Data Types

GraphQL is a powerful and flexible query language that allows clients to request exactly the data they need from APIs. One of the ways it achieves this flexibility is through custom scalar types, which enable you to define your own data types and their associated validation and serialization logic. In this blog post, we'll explore how to create custom GraphQL scalars to handle advanced data types, taking a step-by-step approach that's perfect for beginners. Along the way, we'll provide code examples and explanations to help you gain a deeper understanding of this powerful feature. Finally, we'll wrap up with a "FAQ" section answering some frequently asked questions related to custom scalars.

What are GraphQL Scalars?

Before diving into custom scalars, it's essential to understand what scalars are in the context of GraphQL. Scalars are the basic building blocks of GraphQL's type system, representing the primitive data types used in a schema. GraphQL has five built-in scalar types:

1. Int: A 32-bit integer
2. Float: A floating-point number
3. String: A sequence of characters
4. Boolean: A true or false value
5. ID: A unique identifier, usually a string or integer

These built-in scalar types are adequate for many use cases, but there are situations where you might need to handle more advanced or domain-specific data types. That's where custom scalars come into play.

Creating Custom Scalars

To create a custom scalar, you'll need to define its name, description, and the serialization, parsing, and validation functions that determine how the scalar is processed. Let's walk through the process of creating a custom scalar for handling email addresses.

Step 1: Defining the Custom Scalar

First, we need to define the custom scalar in our GraphQL schema. In this example, we'll create an EmailAddress scalar:

scalar EmailAddress

This simple declaration informs the GraphQL server that we have a new scalar type called EmailAddress.

Step 2: Implementing the Custom Scalar

Now that we have our custom scalar defined in the schema, we need to implement the associated serialization, parsing, and validation logic. For this example, we'll use JavaScript and the popular graphql library. Start by installing the package:

npm install graphql

Next, create a new JavaScript file to implement the custom scalar. We'll start by importing the necessary dependencies:

const { GraphQLScalarType } = require('graphql');
const { Kind } = require('graphql/language');

We'll also need a regular expression to validate email addresses. While there are many possible patterns, we'll use a simplified one for this example:

const EMAIL_REGEX = /^[\w-]+(\.[\w-]+)*@([\w-]+\.)+[a-zA-Z]{2,7}$/;

Now we can implement the EmailAddress scalar using the GraphQLScalarType class:

const EmailAddress = new GraphQLScalarType({
  name: 'EmailAddress',
  description: 'A valid email address',

  serialize(value) {
    if (typeof value !== 'string') {
      throw new TypeError(`Expected a string, but received: ${typeof value}`);
    }

    if (!EMAIL_REGEX.test(value)) {
      throw new TypeError(`Invalid email address: ${value}`);
    }

    return value;
  },

  parseValue(value) {
    if (typeof value !== 'string') {
      throw new TypeError(`Expected a string, but received: ${typeof value}`);
    }

    if (!EMAIL_REGEX.test(value)) {
      throw new TypeError(`Invalid email address: ${value}`);
    }

    return value;
  },

  parseLiteral(ast) {
    if (ast.kind !== Kind.STRING) {
      throw new TypeError(`Expected a string, but received: ${ast.kind}`);
    }

    if (!EMAIL_REGEX.test(ast.value)) {
      throw new TypeError(`Invalid email address: ${ast.value}`);
    }

    return ast.value;
  },
});

module.exports = { EmailAddress };

In the code above, we created a new GraphQLScalarType instance with three functions:

• serialize: This function is responsible for converting the internal representation of the scalar (in this case, a string) to the serialized form that will be sent to the client. We check if the value is a string and if it matches the email regex pattern.
• parseValue: This function is responsible for converting the serialized form of the scalar (received from the client) to the internal representation used by the server. We perform the same validation as in the serialize function.
• parseLiteral: This function is used when the scalar appears in a GraphQL query literal. It receives an AST node and performs similar validation as the other two functions.

Now that we have our custom EmailAddress scalar implemented, we can use it in our GraphQL schema and resolvers.

Step 3: Using the Custom Scalar in the Schema and Resolvers

First, import the EmailAddress scalar in your schema file and include it in your type definitions:

const { EmailAddress } = require('./path/to/EmailAddress');

const typeDefs = `
  scalar EmailAddress

  type User {
    id: ID!
    name: String!
    email: EmailAddress!
  }

  type Query {
    getUser(id: ID!): User
  }
`;

Then, in your resolvers file, make sure to include the EmailAddress scalar:

const { EmailAddress } = require('./path/to/EmailAddress');

const resolvers = {
  EmailAddress,
  Query: {
    getUser: (parent, args, context) => {
      // Fetch user data from your data source, e.g., a database
      // The email address will be automatically validated using the custom scalar
    },
  },
};

With these changes, the EmailAddress custom scalar is now fully integrated into your GraphQL schema and will automatically validate email addresses in both input and output data.

FAQ

Q: When should I use custom scalars in my GraphQL schema?

A: Custom scalars are useful when you need to handle data types that are not covered by GraphQL's built-in scalar types or when you want to enforce specific validation and serialization rules for a particular type of data. Examples of custom scalars include email addresses, URLs, dates, and monetary values.

Q: How do I handle errors and validation with custom scalars?

A: When implementing custom scalars, you should throw an error in the serialize, parseValue, and parseLiteral functions if the provided value does not meet your validation requirements. This will ensure that invalid data is not processed by your GraphQL server.

Q: Can I use custom scalars with other GraphQL libraries and languages?

A: Yes, most GraphQL libraries and languages support custom scalars. The implementation details may vary, but the overall concept is the same: you'll need to define the custom scalar in your schema and provide the associated serialization, parsing, and validation logic.

Q: Can I use third-party custom scalar libraries?

A: Yes, there are many third-party libraries that provide custom scalars for common data types, such as dates and URLs. Using these libraries can save you time and ensure that your custom scalars are implemented correctly. However, make sure to choose a reputable library and verify that it meets your specific requirements.