GraphQL - problem with the code from the tutorial page, it pops up an error "Expected undefined to be a GraphQL schema."

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05-10-2024

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GraphQL: "Expected undefined to be a GraphQL schema" - Troubleshooting a Common Error

Have you encountered the dreaded "Expected undefined to be a GraphQL schema" error while working with GraphQL? This frustrating error often arises when you're setting up your GraphQL server, and it can leave you feeling lost. Let's break down the issue and explore how to fix it.

Understanding the Error

The error "Expected undefined to be a GraphQL schema" signifies that your GraphQL server is unable to locate or construct a valid schema. This schema is essentially the blueprint that defines the structure and functionality of your GraphQL API. Without a schema, GraphQL has no knowledge of the data you want to query or mutate.

Scenario: A Tutorial Trouble

Let's imagine you're following a GraphQL tutorial and have set up the following code:

const { graphqlHTTP } = require('express-graphql');
const { buildSchema } = require('graphql');

// Define the schema
const schema = buildSchema(`
  type Query {
    hello: String
  }
`);

const root = {
  hello: () => {
    return 'Hello World!';
  },
};

const app = express();
app.use('/graphql', graphqlHTTP({
  schema,
  rootValue: root,
  graphiql: true,
}));

app.listen(4000, () => {
  console.log('Running a GraphQL API server at http://localhost:4000/graphql');
});

After running this code, you're greeted with the dreaded "Expected undefined to be a GraphQL schema" message. Why? The problem lies in the way the buildSchema function is used.

The Missing Link: Execution

The buildSchema function only defines your schema. It doesn't actually execute or build the schema object. To solve this, we need to use the graphql function from the graphql package to execute the schema.

The Fix: Bridging the Gap

Replace the code snippet below:

app.use('/graphql', graphqlHTTP({
  schema,
  rootValue: root,
  graphiql: true,
}));

With the following code, introducing the graphql function for execution:

app.use('/graphql', graphqlHTTP((req, res) => {
  graphql(schema, req.query.query, root, {}, req.query.variables)
    .then(result => res.send(JSON.stringify(result)));
}));

This change tells the GraphQL server to execute the schema using the provided query, root value, and variables. This solves the "Expected undefined to be a GraphQL schema" error, and your GraphQL API should now work as expected.

Key Takeaways

1. Understanding the Schema: The schema is crucial for any GraphQL API; it defines the data your API exposes.
2. Don't Just Define, Execute: Defining your schema (using buildSchema or similar functions) is not enough. You must execute it to make it functional.
3. Leverage the graphql Function: The graphql function from the graphql package is essential for executing your schema and handling queries.

Remember: Carefully reviewing your setup and ensuring proper schema execution is crucial for a smoothly functioning GraphQL API. Happy coding!