Manage Fake Skill

This skill helps you create or update a Fake class for testing purposes. Fakes are test doubles that implement interfaces with configurable behavior.

When to Use This Skill

Use this skill when you need to:

• Create a new Fake implementation for testing an interface
• Update an existing Fake to match interface changes

The skill will generate/update:

• A Fake class that implements the interface
• An Initializer interface for required parameters
• A Builder class for easy test instantiation

Usage

Invoke this skill when the user asks to:

• "Create a fake for [InterfaceName]"
• "Generate a test fake for [InterfaceName]"
• "I need a fake [InterfaceName]"
• "Update the fake for [InterfaceName]"
• "The [InterfaceName] interface changed, update its fake"
• "Fix the Fake[ClassName] to match the interface"

Prerequisites

Before creating/updating a fake:

1. Verify the interface exists - The interface you're faking must already be defined
2. Check for existing fake - Use Glob to search for existing Fake[ClassName] files
3. Identify if interface extends IEntity - This affects the pattern used

Create vs Update Decision

If fake file exists: Update mode

• Read the existing fake file
• Read the interface definition
• Compare and identify differences
• Update the fake to match the interface

If fake file does NOT exist: Create mode

• Read the interface definition
• Generate all three components from scratch

Naming Convention

For an interface named IMyClass, create:

• File: FakeMyClass.ts
• Initializer: FakeMyClassInitializer
• Fake class: FakeMyClass
• Builder class: FakeMyClassBuilder

Pattern: Remove the I prefix from the interface name and add Fake prefix.

Required Imports

import { getMockingFunction } from '@zdr-tools/zdr-testing-tools';
import { FakeEntity, FakeEntityBuilder, FakesFactory, type IFakeEntityInitialData } from '@zdr-tools/zdr-entities/fakes';
import type { IEntity, IPropEventBroker, IReadablePropEventBroker, IEntityCollection, IOrderedEntityCollection, IRestorablePropEventBroker } from '@zdr-tools/zdr-entities';

Structure

1. Initializer Interface

Purpose: Defines all required parameters for creating the fake.

Rules:

• All fields are MANDATORY (no optional fields with ?)
• One field per interface field
• One field per interface method (for return values only, named [methodName]ReturnValue)

Standard Pattern:

export interface FakeMyClassInitializer {
  someField: string;
  anotherField: number;
  someMethodReturnValue: boolean;
}

If interface extends IEntity:

export interface FakeMyClassInitializer extends IFakeEntityInitialData {
  someField: string;
  anotherField: number;
  someMethodReturnValue: boolean;
}

2. Fake Class

Purpose: The actual fake implementation of the interface.

Standard Pattern:

export class FakeMyClass implements IMyClass {
  public someField: string;
  public anotherField: number;

  constructor(private fakeMyClassInitialData: FakeMyClassInitializer) {
    this.someField = fakeMyClassInitialData.someField;
    this.anotherField = fakeMyClassInitialData.anotherField;
  }

  someMethod = getMockingFunction<(arg: string) => boolean>(() => {
    return this.fakeMyClassInitialData.someMethodReturnValue;
  });
}

If interface extends IEntity:

export class FakeMyClass extends FakeEntity implements IMyClass {
  public someField: string;
  public anotherField: number;

  constructor(private fakeMyClassInitialData: FakeMyClassInitializer) {
    super(fakeMyClassInitialData);

    this.someField = fakeMyClassInitialData.someField;
    this.anotherField = fakeMyClassInitialData.anotherField;
  }

  someMethod = getMockingFunction<(arg: string) => boolean>(() => {
    return this.fakeMyClassInitialData.someMethodReturnValue;
  });
}

Method Implementation Rules:

• Use getMockingFunction to wrap methods
• Generic type matches the method signature
• Return value comes from [methodName]ReturnValue field in initializer
• Method arguments are NOT stored in initializer, only return values

3. Builder Class

Purpose: Provides a fluent API for creating fakes with sensible defaults.

Standard Pattern:

export class FakeMyClassBuilder {
  private someField: string = '';
  private anotherField: number = 0;
  private someMethodReturnValue: boolean = false;

  withSomeField(value: string): this {
    this.someField = value;
    return this;
  }

  withAnotherField(value: number): this {
    this.anotherField = value;
    return this;
  }

  withSomeMethodReturnValue(value: boolean): this {
    this.someMethodReturnValue = value;
    return this;
  }

  protected getFakeMyClassInitializer(): FakeMyClassInitializer {
    return {
      someField: this.someField,
      anotherField: this.anotherField,
      someMethodReturnValue: this.someMethodReturnValue,
    };
  }

  build(): FakeMyClass {
    return new FakeMyClass(this.getFakeMyClassInitializer());
  }
}

If interface extends IEntity:

export class FakeMyClassBuilder extends FakeEntityBuilder {
  private someField: string = '';
  private anotherField: number = 0;
  private someMethodReturnValue: boolean = false;

  withSomeField(value: string): this {
    this.someField = value;
    return this;
  }

  withAnotherField(value: number): this {
    this.anotherField = value;
    return this;
  }

  withSomeMethodReturnValue(value: boolean): this {
    this.someMethodReturnValue = value;
    return this;
  }

  protected getFakeMyClassInitializer(): FakeMyClassInitializer {
    return {
      ...this.getInitialData(), // From FakeEntityBuilder
      someField: this.someField,
      anotherField: this.anotherField,
      someMethodReturnValue: this.someMethodReturnValue,
    };
  }

  build(): FakeMyClass {
    return new FakeMyClass(this.getFakeMyClassInitializer());
  }
}

Default Values for Builder Fields

Use these default values for different field types:

Type	Default Value
Fields that can be undefined	undefined
string	''
number	0
boolean	false
IReadablePropEventBroker<T>	FakesFactory.createReadablePropEventBroker<T>(defaultValueForT)
IPropEventBroker<T>	FakesFactory.createPropEventBroker<T>(defaultValueForT)
IRestorablePropEventBroker<T>	FakesFactory.createFakeRestorablePropEventBroker<T>(defaultValueForT)
IEntityCollection<T>	FakesFactory.createEntityCollection<T>([])
IOrderedEntityCollection<T>	FakesFactory.createOrderedEntityCollection<T>([])
Entity/Model types	new Fake[EntityName]Builder().build()

EventBroker Special Handling

For EventBroker fields, create TWO "with" methods:

// Field definition
private name: IPropEventBroker<string> = FakesFactory.createPropEventBroker<string>('');

// Method 1: Accept the full EventBroker
withName(value: IPropEventBroker<string>): this {
  this.name = value;
  return this;
}

// Method 2: Accept just the value (convenience method)
withNameValue(value: string): this {
  return this.withName(FakesFactory.createPropEventBroker<string>(value));
}

File Location and Export Structure

IMPORTANT: All fakes MUST be organized in a /fakes folder at the package root level (sibling to /src):

Directory Structure

packages/
  my-package/
    src/
      IMyClass.ts        # Interface definitions
      MyClass.ts
    fakes/               # Sibling to src, NOT inside src
      index.ts           # Exports all fakes
      FakeMyClass.ts     # Fake implementation
      FakeOtherClass.ts

Rules

1. Fakes folder location: Always create fakes in /fakes/ at the package root (sibling to /src)

   • If interface is in packages/my-package/src/IMyClass.ts
   • Create fake in packages/my-package/fakes/FakeMyClass.ts (NOT in src/fakes/)

2. Index file: The /fakes folder MUST contain an index.ts file that exports all fakes

   • If index.ts doesn't exist, create it
   • Export pattern: export * from './FakeMyClass';

3. After creating/updating a fake: Always add/verify the export in /fakes/index.ts

Example index.ts

// fakes/index.ts (at package root, sibling to src/)
export * from './FakeReport';
export * from './FakeUser';
export * from './FakeWorkspace';

Workflow

Create Workflow (No existing fake)

1. Identify the interface - Ask user which interface to fake if not clear
2. Locate the interface file - Use Glob to find the interface definition
3. Read the interface - Get all fields and methods
4. Check if interface extends IEntity - This affects the pattern used
5. Ensure /fakes folder exists - Check if /fakes/ directory exists at package root (sibling to /src), create if needed
6. Create the fake file in /fakes/FakeMyClass.ts with all three components:
   • Initializer interface
   • Fake class
   • Builder class
7. Update the exports index:
   • Check if /fakes/index.ts exists, create if it doesn't
   • Add export line: export * from './FakeMyClass';
   • Keep exports alphabetically sorted

Update Workflow (Fake exists)

1. Identify the interface and fake - Determine which interface/fake to update
2. Read both files:
   • Read the interface definition
   • Read the existing fake file
3. Compare and identify changes:
   • Added fields - Add to initializer, fake class constructor, and builder (with default value and with* method)
   • Removed fields - Remove from all three components
   • Changed field types - Update type in all three components and adjust default value in builder
   • Added methods - Add [methodName]ReturnValue to initializer, add method implementation in fake class with getMockingFunction, add field and with* method to builder
   • Removed methods - Remove [methodName]ReturnValue and all related code
   • Changed method signatures - Update the generic type in getMockingFunction and the return value type
4. Apply updates using Edit tool:
   • Update the Initializer interface
   • Update the Fake class fields, constructor, and methods
   • Update the Builder class fields, with* methods, and the protected getter method
5. Verify completeness - Ensure all interface members are represented in the fake

Update Guidelines

When updating an existing fake:

• Preserve existing structure - Don't rewrite the entire file, use Edit tool for targeted changes
• Maintain consistency - Follow the same patterns used in the existing fake
• Keep default values sensible - When adding new builder fields, use appropriate defaults from the table below
• Handle EventBrokers correctly - If adding an EventBroker field, remember to create BOTH with* methods
• Check inheritance - If the interface extends IEntity and the fake doesn't extend FakeEntity, this is a structural change that may require a larger refactor

Common Update Scenarios

Scenario 1: Adding a New Field

Interface change:

export interface IReport extends IEntity {
  title: string;
  status: string;
  description: string; // NEW FIELD
}

Required updates:

1. Add to FakeReportInitializer: description: string;
2. Add to FakeReport class: public description: string;
3. Add to constructor: this.description = fakeReportInitialData.description;
4. Add to FakeReportBuilder: private description: string = '';
5. Add builder method:

withDescription(value: string): this {
  this.description = value;
  return this;
}

6. Add to builder's getFakeReportInitializer() return object: description: this.description,

Scenario 2: Adding a New Method

Interface change:

export interface IReport extends IEntity {
  // ... existing fields
  validate(): Promise<boolean>; // NEW METHOD
}

Required updates:

1. Add to FakeReportInitializer: validateReturnValue: Promise<boolean>;
2. Add to FakeReport class:

validate = getMockingFunction<() => Promise<boolean>>(() => {
  return this.fakeReportInitialData.validateReturnValue;
});

3. Add to FakeReportBuilder: private validateReturnValue: Promise<boolean> = Promise.resolve(false);
4. Add builder method:

withValidateReturnValue(value: Promise<boolean>): this {
  this.validateReturnValue = value;
  return this;
}

5. Add to builder's getFakeReportInitializer() return object: validateReturnValue: this.validateReturnValue,

Scenario 3: Removing a Field

Interface change:

export interface IReport extends IEntity {
  title: string;
  // status: string; // REMOVED
}

Required updates:

1. Remove from FakeReportInitializer: status: string;
2. Remove from FakeReport class: public status: string;
3. Remove from constructor: this.status = fakeReportInitialData.status;
4. Remove from FakeReportBuilder: private status: string = '';
5. Remove builder method: withStatus(...)
6. Remove from builder's getFakeReportInitializer() return object: status: this.status,

Scenario 4: Changing a Field Type

Interface change:

export interface IReport extends IEntity {
  title: string;
  status: ReportStatus; // Changed from string to enum/type
}

Required updates:

1. Update in FakeReportInitializer: status: ReportStatus;
2. Update in FakeReport class: public status: ReportStatus;
3. Constructor assignment stays the same: this.status = fakeReportInitialData.status;
4. Update in FakeReportBuilder with appropriate default: private status: ReportStatus = ReportStatus.Draft;
5. Update builder method parameter: withStatus(value: ReportStatus): this
6. Builder's getter stays the same: status: this.status,

Scenario 5: Adding an EventBroker Field

Interface change:

export interface IReport extends IEntity {
  title: IPropEventBroker<string>; // Changed from string to EventBroker
}

Required updates:

1. Update in FakeReportInitializer: title: IPropEventBroker<string>;
2. Update in FakeReport class: public title: IPropEventBroker<string>;
3. Constructor assignment stays the same: this.title = fakeReportInitialData.title;
4. Update in FakeReportBuilder: private title: IPropEventBroker<string> = FakesFactory.createPropEventBroker<string>('');
5. Add/update TWO builder methods:

withTitle(value: IPropEventBroker<string>): this {
  this.title = value;
  return this;
}

withTitleValue(value: string): this {
  return this.withTitle(FakesFactory.createPropEventBroker<string>(value));
}

6. Builder's getter stays the same: title: this.title,

Example Reference

See examples.md in the same directory as this skill for complete working examples.

Important Notes

File Organization (CRITICAL)

• All fakes MUST be in /fakes/ directory at package root (sibling to /src, NOT inside /src)
• All fakes MUST be exported from /fakes/index.ts
• After creating or updating any fake, verify the export exists in index.ts
• Keep exports in index.ts alphabetically sorted for maintainability

General Patterns

• All initializer fields are MANDATORY (never use optional ?)
• Methods only track return values, not arguments
• Use getMockingFunction for all method implementations
• Builder fields should have sensible defaults
• EventBroker fields need two "with" methods
• Always return this from builder "with" methods for chaining
• Protected getFake[ClassName]Initializer() returns the initializer object
• Public build() creates the fake instance

When Updating

• Always use the Edit tool for updates, not Write (which overwrites the entire file)
• Update all three components (Initializer, Fake class, Builder) for consistency
• When adding fields/methods, follow the exact same pattern as existing ones
• Check for TypeScript errors after updates to ensure completeness
• If the interface structure changed significantly (e.g., now extends IEntity), consider regenerating instead of updating
• Verify the export still exists in /fakes/index.ts after updates