graphql-resolvers

# GraphQL Resolvers

Apply resolver implementation patterns to create efficient, maintainable
GraphQL servers. This skill covers resolver function signatures,
execution chains, context management, DataLoader patterns, async
handling, authentication, and testing strategies.

## Resolver Function Signature

Every resolver function receives four arguments: parent, args, context,
and info. Understanding these arguments is fundamental to writing
effective resolvers.

```typescript
type ResolverFn = (
  parent: any,
  args: any,
  context: any,
  info: GraphQLResolveInfo
) => any;

const resolvers = {
  Query: {
    // parent: root value (usually undefined for Query)
    // args: arguments passed to the query
    // context: shared context object
    // info: execution information
    user: async (parent, args, context, info) => {
      const { id } = args;
      const { dataSources, user } = context;

      // Use context to access data sources and auth info
      return dataSources.userAPI.getUserById(id);
    },

    posts: async (parent, args, context, info) => {
      const { limit, offset } = args;

      // Access requested fields from info
      const fields = info.fieldNodes[0].selectionSet.selections
        .map(s => s.name.value);

      return context.dataSources.postAPI.getPosts({
        limit,
        offset,
        fields
      });
    }
  }
};
```

## Field Resolvers

Field resolvers define how to resolve individual fields on a type. The
parent argument contains the resolved parent object.

```typescript
const resolvers = {
  Query: {
    user: async (_, { id }, { dataSources }) => {
      return dataSources.userAPI.getUserById(id);
    }
  },

  User: {
    // Field resolver for computed field
    fullName: (parent) => {
      return `${parent.firstName} ${parent.lastName}`;
    },

    // Field resolver for related data
    posts: async (parent, args, { dataSources }) => {
      // parent.id available from parent User object
      return dataSources.p