---
title: graphql/type
---

{/* title can be removed in Nextra 4, since sidebar title will take from first h1 */}

# `graphql/type`

The `graphql/type` module is responsible for defining GraphQL types and schema. You can import either from the `graphql/type` module, or from the root `graphql` module. For example:

```js
import { GraphQLSchema } from 'graphql';
```

## Overview

### Schema

<ul className="apiIndex">
  <li>
    <a href="#graphqlschema">
      `class GraphQLSchema` A representation of the capabilities of a GraphQL
      Server.
    </a>
  </li>
</ul>

### Definitions

<ul className="apiIndex">
  <li>
    <a href="#graphqlscalartype">
      `class GraphQLScalarType` A scalar type within GraphQL.
    </a>
  </li>
  <li>
    <a href="#graphqlobjecttype">
      `class GraphQLObjectType` An object type within GraphQL that contains
      fields.
    </a>
  </li>
  <li>
    <a href="#graphqlinterfacetype">
      `class GraphQLInterfaceType` An interface type within GraphQL that defines
      fields implementations will contain.
    </a>
  </li>
  <li>
    <a href="#graphqluniontype">
      `class GraphQLUnionType` A union type within GraphQL that defines a list
      of implementations.
    </a>
  </li>
  <li>
    <a href="#graphqlenumtype">
      `class GraphQLEnumType` An enum type within GraphQL that defines a list of
      valid values.
    </a>
  </li>
  <li>
    <a href="#graphqlinputobjecttype">
      `class GraphQLInputObjectType` An input object type within GraphQL that
      represents structured inputs.
    </a>
  </li>
  <li>
    <a href="#graphqllist">
      `class GraphQLList` A type wrapper around other types that represents a
      list of those types.
    </a>
  </li>
  <li>
    <a href="#graphqlnonnull">
      `class GraphQLNonNull` A type wrapper around other types that represents a
      non-null version of those types.
    </a>
  </li>
</ul>

### Predicates

<ul className="apiIndex">
  <li>
    <a href="#isinputtype">
      `function isInputType` Returns if a type can be used as input types for
      arguments and directives.
    </a>
  </li>
  <li>
    <a href="#isoutputtype">
      `function isOutputType` Returns if a type can be used as output types as
      the result of fields.
    </a>
  </li>
  <li>
    <a href="#isleaftype">
      `function isLeafType` Returns if a type can be a leaf value in a response.
    </a>
  </li>
  <li>
    <a href="#iscompositetype">
      `function isCompositeType` Returns if a type can be the parent context of
      a selection set.
    </a>
  </li>
  <li>
    <a href="#isabstracttype">
      `function isAbstractType` Returns if a type is a combination of object
      types.
    </a>
  </li>
</ul>

### Un-modifiers

<ul className="apiIndex">
  <li>
    <a href="#getnullabletype">
      `function getNullableType` Strips any non-null wrappers from a type.
    </a>
  </li>
  <li>
    <a href="#getnamedtype">
      `function getNamedType` Strips any non-null or list wrappers from a type.
    </a>
  </li>
</ul>

### Scalars

<ul className="apiIndex">
  <li>
    <a href="#graphqlint">
      `const GraphQLInt` A scalar type representing integers.
    </a>
  </li>
  <li>
    <a href="#graphqlfloat">
      `const GraphQLFloat` A scalar type representing floats.
    </a>
  </li>
  <li>
    <a href="#graphqlstring">
      `const GraphQLString` A scalar type representing strings.
    </a>
  </li>
  <li>
    <a href="#graphqlboolean">
      `const GraphQLBoolean` A scalar type representing booleans.
    </a>
  </li>
  <li>
    <a href="#graphqlid">`const GraphQLID` A scalar type representing IDs.</a>
  </li>
</ul>

## Schema

### GraphQLSchema

```ts
class GraphQLSchema {
  constructor(config: GraphQLSchemaConfig);
}

type GraphQLSchemaConfig = {
  query: GraphQLObjectType;
  mutation?: GraphQLObjectType;
};
```

A Schema is created by supplying the root types of each type of operation,
query and mutation (optional). A schema definition is then supplied to the
validator and executor.

#### Example

```js
const MyAppSchema = new GraphQLSchema({
  query: MyAppQueryRootType,
  mutation: MyAppMutationRootType,
});
```

## Definitions

### GraphQLScalarType

```ts
class GraphQLScalarType<InternalType, ExternalType> {
  constructor(config: GraphQLScalarTypeConfig<InternalType, ExternalType>);
}

type GraphQLScalarTypeConfig<InternalType, ExternalType> = {
  name: string;
  description?: string;
  specifiedByURL?: Maybe<string>;
  serialize: (outputValue: unknown) => ExternalType;
  parseValue?: (inputValue: unknown) => InternalType;
  parseLiteral?: (
    valueAST: Value,
    variables?: Maybe<Record<string, unknown>>,
  ) => InternalType;
};
```

The leaf values of any request and input values to arguments are
Scalars (or Enums) and are defined with a name and a series of serialization
functions used to ensure validity.

#### Example

```js
const OddType = new GraphQLScalarType({
  name: 'Odd',
  // Can be used to link to a specification
  // for this scalar, for instance the JSON
  // specification.
  specifiedByURL: '',
  description:
    'This custom scalar will only return a value if the passed in value is an odd integer, when it's not it will return null.'
  serialize: (outputValue) => {
    // This function gets called for response-data, the application returns data
    // for a property and in the schema we see that this value has the "Odd" type.
   return typeof outputValue === 'number' && outputValue % 2 === 1 ? value : null;
  },
  parseValue: (inputValue) => {
    // This function gets called for input-data, i.e. variables being passed in
    return typeof inputValue === 'number' && outputValue % 2 === 1 ? value : null;
  },
  parseLiteral(ast) {
    // This function gets called when the value is passed in as a literal on the
    // Executable GraphQL Document
    if (ast.kind === Kind.INT) {
      return oddValue(parseInt(ast.value, 10));
    }
    return null;
  },
});
```

### GraphQLObjectType

```ts
class GraphQLObjectType {
  constructor(config: GraphQLObjectTypeConfig);
}

type GraphQLObjectTypeConfig = {
  name: string;
  interfaces?: GraphQLInterfacesThunk | GraphQLInterfaceType[];
  fields: GraphQLFieldConfigMapThunk | GraphQLFieldConfigMap;
  isTypeOf?: (value: any, info?: GraphQLResolveInfo) => boolean;
  description?: string;
};

type GraphQLInterfacesThunk = () => Array<GraphQLInterfaceType>;

type GraphQLFieldConfigMapThunk = () => GraphQLFieldConfigMap;

// See below about resolver functions.
type GraphQLFieldResolveFn = (
  source?: any,
  args?: { [argName: string]: any },
  context?: any,
  info?: GraphQLResolveInfo,
) => any;

type GraphQLResolveInfo = {
  fieldName: string;
  fieldNodes: Array<Field>;
  returnType: GraphQLOutputType;
  parentType: GraphQLCompositeType;
  schema: GraphQLSchema;
  fragments: { [fragmentName: string]: FragmentDefinition };
  rootValue: any;
  operation: OperationDefinition;
  variableValues: { [variableName: string]: any };
};

type GraphQLFieldConfig = {
  type: GraphQLOutputType;
  args?: GraphQLFieldConfigArgumentMap;
  resolve?: GraphQLFieldResolveFn;
  deprecationReason?: string;
  description?: string;
};

type GraphQLFieldConfigArgumentMap = {
  [argName: string]: GraphQLArgumentConfig;
};

type GraphQLArgumentConfig = {
  type: GraphQLInputType;
  defaultValue?: any;
  description?: string;
};

type GraphQLFieldConfigMap = {
  [fieldName: string]: GraphQLFieldConfig;
};
```

Almost all of the GraphQL types you define will be object types. Object types
have a name, but most importantly describe their fields.

When two types need to refer to each other, or a type needs to refer to
itself in a field, you can use a function expression (aka a closure or a
thunk) to supply the fields lazily.

Note that resolver functions are provided the `source` object as the first parameter.
However, if a resolver function is not provided, then the default resolver is
used, which looks for a method on `source` of the same name as the field. If found,
the method is called with `(args, context, info)`. Since it is a method on `source`,
that value can always be referenced with `this`.

#### Examples

```js
const AddressType = new GraphQLObjectType({
  name: 'Address',
  fields: {
    street: { type: GraphQLString },
    number: { type: GraphQLInt },
    formatted: {
      type: GraphQLString,
      resolve(obj) {
        return obj.number + ' ' + obj.street;
      },
    },
  },
});

const PersonType = new GraphQLObjectType({
  name: 'Person',
  fields: () => ({
    name: { type: GraphQLString },
    bestFriend: { type: PersonType },
  }),
});
```

### GraphQLInterfaceType

```ts
class GraphQLInterfaceType {
  constructor(config: GraphQLInterfaceTypeConfig);
}

type GraphQLInterfaceTypeConfig = {
  name: string;
  fields: GraphQLFieldConfigMapThunk | GraphQLFieldConfigMap;
  resolveType?: (value: any, info?: GraphQLResolveInfo) => GraphQLObjectType;
  description?: string;
};
```

When a field can return one of a heterogeneous set of types, a Interface type
is used to describe what types are possible, what fields are in common across
all types, as well as a function to determine which type is actually used
when the field is resolved.

#### Example

```js
const EntityType = new GraphQLInterfaceType({
  name: 'Entity',
  fields: {
    name: { type: GraphQLString },
  },
});
```

### GraphQLUnionType

```ts
class GraphQLUnionType {
  constructor(config: GraphQLUnionTypeConfig);
}

type GraphQLUnionTypeConfig = {
  name: string;
  types: GraphQLObjectsThunk | GraphQLObjectType[];
  resolveType?: (value: any, info?: GraphQLResolveInfo) => GraphQLObjectType;
  description?: string;
};

type GraphQLObjectsThunk = () => GraphQLObjectType[];
```

When a field can return one of a heterogeneous set of types, a Union type
is used to describe what types are possible as well as providing a function
to determine which type is actually used when the field is resolved.

### Example

```js
const PetType = new GraphQLUnionType({
  name: 'Pet',
  types: [DogType, CatType],
  resolveType(value) {
    if (value instanceof Dog) {
      return DogType;
    }
    if (value instanceof Cat) {
      return CatType;
    }
  },
});
```

### GraphQLEnumType

```ts
class GraphQLEnumType {
  constructor(config: GraphQLEnumTypeConfig);
}

type GraphQLEnumTypeConfig = {
  name: string;
  values: GraphQLEnumValueConfigMap;
  description?: string;
};

type GraphQLEnumValueConfigMap = {
  [valueName: string]: GraphQLEnumValueConfig;
};

type GraphQLEnumValueConfig = {
  value?: any;
  deprecationReason?: string;
  description?: string;
};

type GraphQLEnumValueDefinition = {
  name: string;
  value?: any;
  deprecationReason?: string;
  description?: string;
};
```

Some leaf values of requests and input values are Enums. GraphQL serializes
Enum values as strings, however internally Enums can be represented by any
kind of type, often integers.

Note: If a value is not provided in a definition, the name of the enum value
will be used as its internal value.

#### Example

```js
const RGBType = new GraphQLEnumType({
  name: 'RGB',
  values: {
    RED: { value: 0 },
    GREEN: { value: 1 },
    BLUE: { value: 2 },
  },
});
```

### GraphQLInputObjectType

```ts
class GraphQLInputObjectType {
  constructor(config: GraphQLInputObjectConfig);
}

type GraphQLInputObjectConfig = {
  name: string;
  fields:
    | GraphQLInputObjectConfigFieldMapThunk
    | GraphQLInputObjectConfigFieldMap;
  description?: string;
};

type GraphQLInputObjectConfigFieldMapThunk =
  () => GraphQLInputObjectConfigFieldMap;

type GraphQLInputObjectFieldConfig = {
  type: GraphQLInputType;
  defaultValue?: any;
  description?: string;
};

type GraphQLInputObjectConfigFieldMap = {
  [fieldName: string]: GraphQLInputObjectFieldConfig;
};

type GraphQLInputObjectField = {
  name: string;
  type: GraphQLInputType;
  defaultValue?: any;
  description?: string;
};

type GraphQLInputObjectFieldMap = {
  [fieldName: string]: GraphQLInputObjectField;
};
```

An input object defines a structured collection of fields which may be
supplied to a field argument.

Using `NonNull` will ensure that a value must be provided by the query

#### Example

```js
const GeoPoint = new GraphQLInputObjectType({
  name: 'GeoPoint',
  fields: {
    lat: { type: new GraphQLNonNull(GraphQLFloat) },
    lon: { type: new GraphQLNonNull(GraphQLFloat) },
    alt: { type: GraphQLFloat, defaultValue: 0 },
  },
});
```

### GraphQLList

```ts
class GraphQLList {
  constructor(type: GraphQLType);
}
```

A list is a kind of type marker, a wrapping type which points to another
type. Lists are often created within the context of defining the fields of
an object type.

#### Example

```js
const PersonType = new GraphQLObjectType({
  name: 'Person',
  fields: () => ({
    parents: { type: new GraphQLList(PersonType) },
    children: { type: new GraphQLList(PersonType) },
  }),
});
```

### GraphQLNonNull

```ts
class GraphQLNonNull {
  constructor(type: GraphQLType);
}
```

A non-null is a kind of type marker, a wrapping type which points to another
type. Non-null types enforce that their values are never null and can ensure
an error is raised if this ever occurs during a request. It is useful for
fields which you can make a strong guarantee on non-nullability, for example
usually the id field of a database row will never be null.

#### Example

```js
const RowType = new GraphQLObjectType({
  name: 'Row',
  fields: () => ({
    id: { type: new GraphQLNonNull(String) },
  }),
});
```

## Predicates

### isInputType

```js
function isInputType(type: GraphQLType): boolean
```

These types may be used as input types for arguments and directives.

### isOutputType

```ts
function isOutputType(type: GraphQLType): boolean;
```

These types may be used as output types as the result of fields

### isLeafType

```ts
function isLeafType(type: GraphQLType): boolean;
```

These types may describe types which may be leaf values

### isCompositeType

```ts
function isCompositeType(type: GraphQLType): boolean;
```

These types may describe the parent context of a selection set

### isAbstractType

```ts
function isAbstractType(type: GraphQLType): boolean;
```

These types may describe a combination of object types

## Un-modifiers

### getNullableType

```ts
function getNullableType(type: GraphQLType): GraphQLNullableType;
```

If a given type is non-nullable, this strips the non-nullability and
returns the underlying type.

### getNamedType

```ts
function getNamedType(type: GraphQLType): GraphQLNamedType;
```

If a given type is non-nullable or a list, this repeated strips the
non-nullability and list wrappers and returns the underlying type.

## Scalars

### GraphQLInt

```ts
let GraphQLInt: GraphQLScalarType;
```

A `GraphQLScalarType` that represents an int.

### GraphQLFloat

```ts
let GraphQLFloat: GraphQLScalarType;
```

A `GraphQLScalarType` that represents a float.

### GraphQLString

```ts
let GraphQLString: GraphQLScalarType;
```

A `GraphQLScalarType` that represents a string.

### GraphQLBoolean

```ts
let GraphQLBoolean: GraphQLScalarType;
```

A `GraphQLScalarType` that represents a boolean.

### GraphQLID

```ts
let GraphQLID: GraphQLScalarType;
```

A `GraphQLScalarType` that represents an ID.
