Creating Custom Data Classes

You can use FP’s Literal class for successful data, but has many limitations.

To go beyond the default Literal class, you can define a custom class that extends DataFP (an abstract class from FP).

Example class created by extending DataFP

import { ApiProperty } from "@nestjs/swagger";
import { Expose } from "class-transformer";
import { Equals, IsIn, IsJWT, IsString } from "class-validator";
import { DataFP } from "./DataFP";

export class LoginResponse extends DataFP<"LoginResponse", 200 | 201> {
  public static readonly kind = "LoginResponse"; (1)

  @ApiProperty() (2)
  @Equals("LoginResponse")
  @Expose()
  public override readonly kind: "LoginResponse"; (3)

  @ApiProperty()
  @IsIn([200, 201])
  @Expose()
  public override readonly status: 200 | 201; (3)

  @ApiProperty()
  @IsString()
  @Expose()
  public readonly id: string; (4)

  @ApiProperty()
  @IsJWT()
  @Expose()
  public readonly jwt: string; (4)

  public constructor(
    id: string,
    jwt: string,
    newRegistration: boolean = false,
  ) {
    super();
    this.kind = "LoginResponse";
    this.status = newRegistration ? 201 : 200;
    this.id = id;
    this.jwt = jwt;
  }
}

1	Defining a static `kind` property is used in some utilities, but is not required.
2	`DataFP` does not attach `ApiProperty`, `Expose`, or any validation decorators (unlike other `FP` classes) - if using OpenAPI or fp-interceptor you should declare `ApiProperty` and `Expose`.
3	You must define `kind` and `status` fields.
4	You may define any additional fields to represent your data, such as `id` and `jwt`.

Now the properties are exposed directly in the returned object, no need to go through data:

Example usage of custom class extending DataFP

const response = new LoginResponse("admin", "[jwt contents]");
expect(response.id).toBe("admin");

If an error might be returned, you can check that the value is a LoginResponse before accessing fields, or use a transformation like map or chain.

async function performLogin(
  username: string,
  password: string,
): Promise<LoginResponse | UnexpectedError<LoginResponse>> {
  return response;
}

const login = await performLogin("admin", "top-secret-password");
if(login.hasData) {
  console.log(`Success!  JWT: ${login.jwt}`);
} else {
  console.log("Failed login.");
}

Common FP Instances