@cats-cradle/faker-factory

FakerFactory creates fakes of classes using class-validator decorators for testing purposes.

It creates a faker object by first inferring each property's type based the class-validator decorator. Next, it changes the object to a schema then it generates fake data for each property. Lastly, the faked object is instantiated and returned.

Getting Started

Create a fake by calling the FakerFactory.

import { FakerFactory } from '@cats-cradle/faker-factory';
import { Person } from './person.ts';

const fakerPerson = await FakerFactory.create<Person>(Person, { passed: true });

// person.ts or any class that uses class-validator decorators
import {
  IsString,
  IsInt,
  IsBoolean,
  IsUUID,
  IsCurrency,
  Min,
  ValidateNested,
} from 'class-validator';
import { Type } from 'class-transformer';

export class Person {
  @IsUuid();
  public id: string;

  @IsString()
  @Length(0,50)
  @IsOptional()
  public firstName: string;

  @IsCurrency()
  public currency: string;

  @IsInt()
  @Min(0)
  public visits: number;

  @IsBoolean()
  public passed: boolean;

  @IsOptional()
  @ValidateNested({ each: true })
  @Type(() => Person)
  public guests: Person[];
}

// result
// console.log(fakerPerson);
{
  "id": "4cb85e06-1060-4bed-8224-14ec39e0dfa9",
  "firstName": "irure in",
  "currency": "887.56",
  "visits": 24235,
  "passed": true
}

Settings

FakerFactory accepts a third optional parameter for settings. Settings are used to change the way the FakerFactory works. Adjusting the settings can be useful for when writing unit tests on optional fields.

Property	Description	Possible Values
probability	probability for faking optional fields	0, ... 0.5, ... 1
optionals	whether to fake optional fields	true, false, undefined

const Person = await FakerFactory.create<Person>(
  Person,
  {},
  { optionals: true },
);

Opinions

In typescript object data can be automatically faked based on either: the property name, property typehint, separate schema, or the decorator schema.

Property names should remain somewhat consistent and fakes can be generated based on them, this is how Intermock works. However, it is unreasonable to maintain such a many-to-many relationship for faking purposes.

Generating values based on property typehinting is less then ideal currently in typescript due to compiled code being type agnostic and the special tsc requirement to add such a feature.

Generating fakes purely based on separate schema such as AJV, JIO, or validatorjs works but it is less readable than a purely decorator based approach.

A class-validator decorators and schema base approach was chosen for the following reasons:

• Decorators are already present when using class-validation.
• Faker data should not just be fake, it should be valid and meet class-validator requirements.
• If a class-validator requires a string length faker data should meet that requirement.
• It is easy to create new class-validator decorators and extend functionality.

References

• F.I.R.S.T Principles of Testing
• Fake vs Mock
• Faking vs Mocking vs Stubbing
• fakerJs
• class-transformer
• class-validator
• class-validator jsonschema