Introduction to TestDouble

TestDouble is a versatile and powerful JavaScript testing library that facilitates the creation of mock objects, spies, and stubs for unit testing. It aims to simplify the testing process by providing an intuitive and flexible API that allows developers to easily replace real objects in their code with test doubles. This ensures isolated and effective testing.

Creating Test Doubles

TestDouble offers a range of APIs to create mocks, stubs, and spies. Below, we delve into these APIs with several examples:

Mocking Functions

  const td = require('testdouble');
  
  // Create a mock function
  const myFunction = td.func('myFunction');

Stubbing Functions

  // Stub a function's return value
  td.when(myFunction(42)).thenReturn('The answer');

Verifying Function Calls

  // Verify the function was called with specified arguments
  td.verify(myFunction(42));

Replacing Real Objects

Replace a real object method with a test double:

  const MyClass = require('./MyClass');
  td.replace(MyClass.prototype, 'method');

Resetting Test Doubles

  // Reset all test doubles
  td.reset();

Comprehensive Example: Todo Application

Let’s build a simple Todo application and test its logic using TestDouble.

Todo Application Code

  // app.js
  class TodoApp {
    constructor() {
      this.todos = [];
    }

    addTodo (title) {
      this.todos.push({ title, completed: false });
    }
    
    completeTodo (index) {
      this.todos[index].completed = true;
    }
    
    getTodos () {
      return this.todos;
    }
  }
  
  module.exports = TodoApp;

Test Suite with TestDouble

  const td = require('testdouble');
  const assert = require('assert');
  const TodoApp = require('./app.js');
  
  describe('TodoApp', () => {
    let app;
    
    beforeEach(() => {
      app = new TodoApp();
    });
  
    it('should add a todo', () => {
      // Mock addTodo method
      td.replace(app, 'addTodo');
      
      app.addTodo = td.func();
      td.when(app.addTodo('Learn TestDouble')).thenReturn();
      
      app.addTodo('Learn TestDouble');
      td.verify(app.addTodo('Learn TestDouble'));
    });
    
    it('should complete a todo', () => {
      app.addTodo = td.func();
      app.completeTodo = td.func();
      
      app.addTodo('Learn TestDouble');
      td.when(app.addTodo('Learn TestDouble')).thenReturn();
      
      app.completeTodo = td.func();
      td.when(app.completeTodo(0)).thenReturn(true);
      
      app.completeTodo(0);
      td.verify(app.completeTodo(0));
    });
    
    it('should get all todos', () => {
      app.addTodo = td.func();
      app.getTodos = td.func();
      
      app.addTodo('Learn TestDouble');
      td.when(app.addTodo('Learn TestDouble')).thenReturn();
      
      app.getTodos();
      assert.deepEqual(app.getTodos(), [{ title: 'Learn TestDouble', completed: false }]);
    });
  });

By effectively utilizing TestDouble APIs such as td.replace, td.when, and td.verify, we can create comprehensive test suites that ensure our code functions as expected while remaining isolated from external dependencies.

Happy Testing!

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