Directives in GraphQL

1. Introduction

GraphQL directives are a powerful feature that allows developers to attach metadata to their schema, enabling conditional behaviors during query execution. They can modify the structure of the execution or how specific fields are resolved.

2. What are Directives?

Directives are special annotations in your schema and queries. They allow you to conditionally include or skip fields and modify the behavior of your queries based on certain conditions. They are defined using the @directiveName syntax.

Some common use cases include:

• Conditional inclusion of fields
• Field transformations
• Debugging and logging

3. Built-in Directives

GraphQL provides built-in directives that you can use:

• @include(if: Boolean): Conditionally include a field based on a boolean expression.
• @skip(if: Boolean): Conditionally skip a field based on a boolean expression.
• @deprecated(reason: String): Marks a field as deprecated, with an optional reason.

Example Usage:

{
  user(id: "1") {
    name
    email @include(if: $showEmail)
    age @skip(if: $hideAge)
  }
}
                

4. Creating Custom Directives

You can create custom directives to add your logic. Here’s how:

1. Define the directive in your schema.
2. Implement the directive logic in your resolver.
3. Use the directive in your queries.

Step-by-Step Example:

1. Define the Directive:

directive @upper on FIELD_DEFINITION

type Query {
  greet(name: String @upper): String
}
                

2. Implement the Directive Logic:

const { SchemaDirectiveVisitor } = require('graphql-tools');

class UpperDirective extends SchemaDirectiveVisitor {
  visitFieldDefinition(field) {
    const { resolve = defaultFieldResolver } = field;
    field.resolve = async function (...args) {
      const result = await resolve.apply(this, args);
      return result.toUpperCase();
    };
  }
}
                

3. Use the Directive in Queries:

{
  greet(name: "world") @upper
}
                

5. Best Practices

When working with directives, keep the following best practices in mind:

• Use built-in directives when possible to reduce complexity.
• Keep custom directives simple and focused on a single responsibility.
• Document your directives for better maintainability.
• Avoid using directives for complex logic that could be handled in resolvers.

6. FAQ