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Scalars

The GraphQL includes the following default types: Int, Float, String, Boolean and ID. However, sometimes you may need to support custom atomic data types (e.g. Date).

Schema first#

In order to define a custom scalar (read more about scalars here), we have to create a type definition and a dedicated resolver as well. Here (as in the official documentation), we’ll take the graphql-type-json package for demonstration purposes. This npm package defines a JSON GraphQL scalar type. Firstly, let's install the package:

$ npm i --save graphql-type-json

Once the package is installed, we have to pass a custom resolver to the forRoot() method:

import * as GraphQLJSON from 'graphql-type-json';

@Module({
  imports: [
    GraphQLModule.forRoot({
      typePaths: ['./**/*.graphql'],
      resolvers: { JSON: GraphQLJSON },
    }),
  ],
})
export class ApplicationModule {}

Now we can use JSON scalar in our type definitions:

scalar JSON

type Foo {
  field: JSON
}

Another form of defining the scalar type is to create a simple class. Let's say that we would like to enhance our schema with the Date type.

import { Scalar, CustomScalar } from '@nestjs/graphql';
import { Kind, ValueNode } from 'graphql';

@Scalar('Date')
export class DateScalar implements CustomScalar<number, Date> {
  description = 'Date custom scalar type';

  parseValue(value: number): Date {
    return new Date(value); // value from the client
  }

  serialize(value: Date): number {
    return value.getTime(); // value sent to the client
  }

  parseLiteral(ast: ValueNode): Date {
    if (ast.kind === Kind.INT) {
      return new Date(ast.value);
    }
    return null;
  }
}

Afterward, we need to register DateScalar as a provider.

@Module({
  providers: [DateScalar],
})
export class CommonModule {}

And now we are able to use Date scalar in our type definitions.

scalar Date

Code first#

In order to create a Date scalar, simply create a new class.

import { Scalar, CustomScalar } from '@nestjs/graphql';
import { Kind, ValueNode } from 'graphql';

@Scalar('Date', type => Date)
export class DateScalar implements CustomScalar<number, Date> {
  description = 'Date custom scalar type';

  parseValue(value: number): Date {
    return new Date(value); // value from the client
  }

  serialize(value: Date): number {
    return value.getTime(); // value sent to the client
  }

  parseLiteral(ast: ValueNode): Date {
    if (ast.kind === Kind.INT) {
      return new Date(ast.value);
    }
    return null;
  }
}

Once it's ready, register DateScalar as a provider.

@Module({
  providers: [DateScalar],
})
export class CommonModule {}

Now you can use Date type in your classes.

@Field()
creationDate: Date;