Using Jasmines Spyon Upon A Private Method

When working with Jasmine for testing your code, you may come across situations where you need to test a private method. While Jasmine doesn't provide built-in support for directly spying on private methods, there is a workaround that involves taking advantage of Jasmine's `spyOn` function.

To spy on a private method using Jasmine, the first step is to expose the private method in some way that allows Jasmine to spy on it. One common approach is to refactor your code slightly to make the private method accessible outside of its original scope. This can be done by either extracting the private method into its own function or by defining a public method that internally calls the private method.

Once you have made the private method accessible, you can use Jasmine's `spyOn` function to create a spy on that method. This allows you to track calls to the private method, inspect arguments passed to it, and even mock its behavior if needed.

Let's walk through an example to illustrate how you can spy on a private method using Jasmine.

Suppose we have a class `Calculator` with a private method `_add` that we want to test:

Javascript

class Calculator {
  _add(a, b) {
    return a + b;
  }

  addNumbers(a, b) {
    return this._add(a, b);
  }
}

To test the `_add` method, we need to make it accessible and spy on it. Here's how you can achieve this using Jasmine:

Javascript

describe('Calculator', () =&gt; {
  it('should spy on private method _add', () =&gt; {
    const calculator = new Calculator();

    spyOn(calculator, '_add').and.callThrough();

    const result = calculator.addNumbers(2, 3);

    expect(calculator._add).toHaveBeenCalled();
    expect(result).toEqual(5);
  });
});

In this example, we create a spy on the `_add` method of the `Calculator` instance using `spyOn(calculator, '_add').and.callThrough()`. The `callThrough()` method ensures that the original implementation of `_add` is still called when the `addNumbers` method is invoked.

By using `expect(calculator._add).toHaveBeenCalled()`, we can verify that the `_add` method has been called during the test. Additionally, we check the result of calling `addNumbers(2, 3)` to validate that our private method behaves as expected.

Remember that while spying on private methods can be useful for testing, it's generally a good practice to focus on testing the public API of your code as much as possible. Private methods are implementation details and can change frequently without affecting the external behavior of your code.

By following these steps and leveraging Jasmine's spy capabilities, you can effectively test private methods in your JavaScript codebase. Happy coding and testing!

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