GraphQL is a powerful and flexible query language that allows you to retrieve data from an API in a structured and efficient way. One of the key features of GraphQL is its ability to retrieve information about the data that is being returned by the API. This is made possible by the use of a special field called __typename.

We’ll take a closer look at what __typename is, why it is significant?

What is __typename?

__typename is a reserved field in GraphQL that returns the name of the object type for a particular GraphQL object. It is used to determine the type of any object in a GraphQL schema.

When you query a GraphQL API, the data that is returned can be of different types. For example, if you query a social media API, you might get data for both users and posts. In this case, the data returned for users and posts will have different fields and different types.

To determine the type of data being returned by the API, you can use the __typename field. This field is automatically added to every GraphQL object and returns the name of the object type for that object.

Why is __typename Significant?

__typename is significant for a number of reasons. First and foremost, it allows you to write more efficient and effective GraphQL queries. When you use __typename in your queries, you can determine the type of the data being returned and query only the fields that are relevant to that type.

For example, consider the following GraphQL query

query {
  posts {
    __typename
    title
    author {
      __typename
      name
      email
    }
  }
}

In this query, we’re querying for posts and their authors. By using __typename, we can determine the type of the author object and query only the fields that are relevant to that type. This can significantly reduce the amount of data that is returned by the API and improve the performance of your application.

Another reason why __typename is significant is that it allows you to write more polymorphic queries. A polymorphic query is one that can return objects of different types depending on the input parameters or other conditions. __typename allows you to determine the type of each object returned and query the appropriate fields based on the object's type.

How to Use __typename in Your GraphQL Queries

Using __typename in your GraphQL queries is easy. Simply add __typename to your query and it will return the name of the object type for each object in the response.

Here’s an example of a query that uses __typename:

query {
  search(query: "GraphQL") {
    __typename
    ... on Repository {
      name
      description
    }
    ... on User {
      username
      bio
    }
  }
}

In this query, we’re searching for either repositories or users that match the query “GraphQL”. By using __typename and the ... on syntax, we can determine the type of each object returned and query only the fields that are relevant to that type.

Polymorphism in GraphQL

Polymorphism refers to the ability of a GraphQL API to return objects of different types in response to a single query. This is a powerful feature that allows you to write more flexible and reusable queries.

Polymorphism is achieved in GraphQL through the use of interfaces and unions. An interface is a way of defining a set of fields that must be implemented by any object that implements that interface. A union is a way of combining multiple object types into a single type.

For example, consider the following GraphQL schema:

interface Node {
  id: ID!
}

type User implements Node {
  id: ID!
  name: String!
  email: String!
}

type Post implements Node {
  id: ID!
  title: String!
  content: String!
}

union SearchResult = User | Post

type Query {
  search(query: String!): [SearchResult!]!
}

In this schema, we have defined an interface called Node that has a single field called id. We have also defined two object types that implement the Node interface: User and Post. Finally, we have defined a union type called SearchResult that can be either a User or a Post.

The Query type has a single field called search that returns an array of SearchResult objects. This allows us to write a single query that can return both users and posts.

Polymorphic Queries in GraphQL

A polymorphic query is a query that can return objects of different types depending on the input parameters or other conditions. Polymorphic queries are useful when you want to write a single query that can handle multiple use cases.

In GraphQL, you can write polymorphic queries by using the ... on syntax with an interface or union type. This syntax allows you to query fields that are specific to a particular object type.

Conclusion

Polymorphism and polymorphic queries are powerful features of GraphQL that allow you to write more flexible and reusable queries. By using interfaces and unions, you can define a set of fields that can be implemented by multiple object types, and by using the ... on syntax, you can query only the fields that are relevant to a particular object type.

__typename is a key feature of polymorphic queries in GraphQL. It allows you to determine the type of each object returned and query the appropriate fields based on the object's type. By using __typename in your queries, you can write more efficient and effective GraphQL queries that can handle multiple use cases.

Thank you
Rohan Joshi