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Understanding the GraphQL Type System

From scalars and custom types to objects and root operations, we explain the basics of the GraphQL type system and how they form a complete GraphQL schema.

GraphQL is undeniably an impressive tool for data querying and manipulation. Central to the GraphQL world is the type system. Understanding the various types and their usage allows you to create effective and precise GraphQL schemas.

Scalar Types

Let’s start with scalar types, which are the fundamental building blocks in a GraphQL schema. Scalar types represent simple, primitive values. GraphQL provides five built-in scalar types:

  • Int: This type represents whole numbers without decimal places, e.g., 42.
  • Float: Float stands for floating-point numbers, i.e., numbers with decimal places, e.g., 3.14.
  • String: This represents strings that hold text, e.g., “Hello, world!”.
  • Boolean: This type has only two possible values, true or false. For example, true.
  • ID: ID stands for a unique identifier. It is always transmitted as a string, even if it is numeric, e.g., “abc123”.

Additionally, you can create custom scalar types to meet specific requirements, such as a “Date” or “Time” type.

Enum Types

The enum type allows you to define a list of fixed values. Enums are useful when you want more precise control over the response. For instance, you could use enums to define the jobs or species of characters in a game.

Non-Null and List Types

In GraphQL, all types are nullable by default, meaning they can be null. To declare a type as non-null, you add an exclamation mark, like “String!”. This ensures that the field always returns a value.

List types, specified within square brackets, represent collections of values of the same type. For example, “[String]” defines a list of strings. You can combine non-null and list types to create a non-empty list of strings, such as “[String]!”.

Object Types

Object types form the “branches” of a GraphQL schema and represent complex data structures. They consist of named fields and the types that those fields resolve to. An object type is defined using the “type” keyword. For instance, you could create a “User” object type containing fields like “name” (String) and “email” (String).

Root Operation Types

In a GraphQL schema, there are three special objects serving as entry points: Query, Mutation, and Subscription. The “Query” type allows data reading, similar to a REST GET request. “Mutation” is for write operations like POST or PUT in REST. “Subscription” enables event tracking and is often used with Websockets.

Field Arguments

Fields in GraphQL objects can accept arguments to precisely control queries. For example, if you want to find a specific user by their ID, you can create a “user” field that expects an ID as an argument.

Interface Types

Interface types allow you to define a shared set of fields that can be implemented by different objects. This is useful when multiple objects have similar fields. For example, different character types (Fighter, Wizard, Healer) could implement a common “Character” interface.

Union Types

Union types allow you to unite multiple different object types into a single type. They are useful when a field can return different types. For example, you could create a “SearchResult” union containing both “User” and “Post” objects.

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