Resolvers

Apollo Server needs to know how to populate data for every field in your schema so that it can respond to requests for that data. To accomplish this, it uses resolvers.

A resolver is a function that's responsible for populating the data for a single field in your schema. It can populate that data in any way you define, such as by fetching data from a back-end database or a third-party API.

If you don't define a resolver for a particular field, Apollo Server automatically defines a default resolver for it.

Defining a resolver

Base syntax

Let's say our server defines the following (very short) schema:

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type Query {
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  numberSix: Int! # Should always return the number 6 when queried
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  numberSeven: Int! # Should always return 7
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}

We want to define resolvers for the numberSix and numberSeven fields of the root Query type so that they always return 6 and 7 when they're queried.

Those resolver definitions look like this:

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const resolvers = {
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  Query: {
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    numberSix() {
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      return 6;
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    },
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    numberSeven() {
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      return 7;
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    }
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  }
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};

As this example shows:

• You define all of your server's resolvers in a single JavaScript object (named resolvers above). This object is called the resolver map.
• The resolver map has top-level fields that correspond to your schema's types (such as Query above).
• Each resolver function belongs to whichever type its corresponding field belongs to.

Handling arguments

Now let's say our server defines the following (slightly longer) schema:

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type User {
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  id: ID!
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  name: String
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}
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type Query {
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  user(id: ID!): User
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}

We want to be able to query the user field to fetch a user by its id.

To achieve this, our server needs access to user data. For this contrived example, assume our server defines the following hardcoded array:

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const users = [
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  {
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    id: '1',
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    name: 'Elizabeth Bennet'
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  },
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  {
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    id: '2',
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    name: 'Fitzwilliam Darcy'
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  }
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];

Now we can define a resolver for the user field, like so:

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const resolvers = {
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  Query: {
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    user(parent, args, context, info) {
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      return users.find(user => user.id === args.id);
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    }
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  }
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}

As this example shows:

• A resolver can optionally accept four positional arguments: (parent, args, context, info).

  Learn more about these arguments

• The args argument is an object that contains all GraphQL arguments that were provided for the field by the GraphQL operation.

Passing resolvers to Apollo Server

After you define all of your resolvers, you pass them to the constructor of ApolloServer (as the resolvers property), along with your schema's definition (as the typeDefs property).

The following example defines a hardcoded data set, a schema, and a resolver map. It then initializes an ApolloServer instance, passing the schema and resolvers to it.

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const { ApolloServer, gql } = require('apollo-server');
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// Hardcoded data store
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const books = [
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  {
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    title: 'The Awakening',
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    author: 'Kate Chopin',
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  },
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  {
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    title: 'City of Glass',
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    author: 'Paul Auster',
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  },
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];
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// Schema definition
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const typeDefs = gql`
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  type Book {
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    title: String
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    author: String
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  }
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  type Query {
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    books: [Book]
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  }
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`;
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// Resolver map
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const resolvers = {
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  Query: {
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    books() {
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      return books;
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    }
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  },
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};
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// Pass schema definition and resolvers to the
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// ApolloServer constructor
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const server = new ApolloServer({ typeDefs, resolvers });
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// Launch the server
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server.listen().then(({ url }) => {
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  console.log(`🚀  Server ready at ${url}`);
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});

Note that you can define your resolvers across as many different files and objects as you want, as long as you merge all of them into a single resolver map that's passed to the ApolloServer constructor.

Resolver chains

Whenever a query asks for a field that contains an object type, the query also asks for at least one field of that object (if it didn't, there would be no reason to include the object in the query). A query always "bottoms out" on fields that contain either a scalar or a list of scalars.

Therefore, whenever Apollo Server resolves a field that contains an object type, it always then resolves one or more fields of that object. Those subfields might in turn also contain object types. Depending on your schema, this object-field pattern can continue to an arbitrary depth, creating what's called a resolver chain.

Example

Let's say our server defines the following schema:

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# A library has a branch and books
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type Library {
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  branch: String!
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  books: [Book!]
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}
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# A book has a title and author
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type Book {
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  title: String!
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  author: Author!
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}
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# An author has a name
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type Author {
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  name: String!
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}
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type Query {
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  libraries: [Library]
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}

Here's a valid query against that schema:

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query GetBooksByLibrary {
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  libraries {
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    books {
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      author {
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        name
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      }
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    }
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  }
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}

The resulting resolver chain for this query matches the hierarchical structure of the query itself:

These resolvers execute in the order shown above, passing their return value to the next resolver in the chain via the parent argument.

Here's a code sample that can resolve the query above with this resolver chain:

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const { ApolloServer, gql } = require('apollo-server');
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const libraries = [
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  {
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    branch: 'downtown'
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  },
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  {
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    branch: 'riverside'
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  },
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];
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// The branch field of a book indicates which library has it in stock
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const books = [
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  {
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    title: 'The Awakening',
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    author: 'Kate Chopin',
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    branch: 'riverside'
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  },
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  {
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    title: 'City of Glass',
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    author: 'Paul Auster',
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    branch: 'downtown'
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  },
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];
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// Schema definition
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const typeDefs = gql`
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# A library has a branch and books
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  type Library {
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    branch: String!
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    books: [Book!]
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  }
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  # A book has a title and author
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  type Book {
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    title: String!
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    author: Author!
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  }
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  # An author has a name
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  type Author {
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    name: String!
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  }
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  # Queries can fetch a list of libraries
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  type Query {
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    libraries: [Library]
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  }
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`;
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// Resolver map
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const resolvers = {
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  Query: {
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    libraries() {
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      // Return our hardcoded array of libraries
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      return libraries;
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    }
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  },
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  Library: {
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    books(parent) {
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      // Filter the hardcoded array of books to only include
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      // books that are located at the correct branch
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      return books.filter(book => book.branch === parent.branch);
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    }
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  },
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  Book: {
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    // The parent resolver (Library.books) returns an object with the
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    // author's name in the "author" field. Return a JSON object containing
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    // the name, because this field expects an object.
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    author(parent) {
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      return {
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        name: parent.author
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      };
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    }
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  }
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  // Because Book.author returns an object with a "name" field,
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  // Apollo Server's default resolver for Author.name will work.
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  // We don't need to define one.
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};
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// Pass schema definition and resolvers to the
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// ApolloServer constructor
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const server = new ApolloServer({ typeDefs, resolvers });
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// Launch the server
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server.listen().then(({ url }) => {
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  console.log(`🚀  Server ready at ${url}`);
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});

If we now update our query to also ask for each book's title:

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query GetBooksByLibrary {
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  libraries {
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    books {
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      title
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      author {
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        name
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      }
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    }
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  }
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}

Then the resolver chain looks like this:

When a chain "diverges" like this, each subchain executes in parallel.

Resolver arguments

Resolver functions are passed four arguments: parent, args, context, and info (in that order).

The context argument

The context argument is useful for passing things that any resolver might need, like authentication scope, database connections, and custom fetch functions. If you're using dataloaders to batch requests across resolvers, you can attach them to the context as well.

Resolvers should never destructively modify the context argument. This ensures consistency across all resolvers and prevents unexpected errors.

To provide a context to your resolvers, add a context initialization function to the ApolloServer constructor. This function is called with every request, so you can set the context based on the request's details (such as HTTP headers).

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// Constructor
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const server = new ApolloServer({
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  typeDefs,
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  resolvers,
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  context: ({ req }) => ({
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    authScope: getScope(req.headers.authorization)
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  })
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}));
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// Example resolver
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(parent, args, context, info) => {
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  if(context.authScope !== ADMIN) throw new AuthenticationError('not admin');
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  // Proceed
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}

Context initialization can be asynchronous, allowing database connections and other operations to complete:

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context: async () => ({
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  db: await client.connect(),
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})
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// Resolver
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(parent, args, context, info) => {
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  return context.db.query('SELECT * FROM table_name');
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}

Return values

A resolver function's return value is treated differently by Apollo Server depending on its type:

Default resolvers

If you don't define a resolver for a particular schema field, Apollo Server defines a default resolver for it (see the source).

The default resolver function uses the following logic:

As an example, consider the following schema excerpt:

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type Book {
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  title: String
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}
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type Author {
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  books: [Book]
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}

If the resolver for the books field returns an array of objects that each contain a title field, then you can use a default resolver for the title field. The default resolver will correctly return parent.title.

Monitoring resolver performance

As with all code, a resolver's performance depends on its logic. It's important to understand which of your schema's fields are computationally expensive or otherwise slow to resolve, so that you can either improve their performance or make sure you only query them when necessary.

Apollo Studio integrates directly with Apollo Server to provide field-level metrics that help you understand the performance of your data graph over time. For more information, see Analyzing performance.

While developing locally, you can provide tracing: true to the ApolloServer constructor to enable a tracing panel:

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const server = new ApolloServer({
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  typeDefs,
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  resolvers,
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  tracing: true,
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});