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TDD - How to go about it? - A quick start example

So, it's only recently I've been doing TDD. I have written unit tests before but those were all tests after writing the actual code. Confusing statement right?

/**

 * @disclaimer
 * I'm just preaching what I practice and some parts of it could be wrong.
 * Please feel free to leave comments if am wrong
 * And I would be happy to stand corrected

*/

What is TDD?

TDD, expanded as Test Driven Development could have this statement as one of the definitions. "TDD is nothing but the art of writing down the business use case one by one and write code based on it paralelly"

It would feel like why do I need to write test for that? We could do the same with pen & paper and write codes. But then, you again circle back to write tests for the same. So why not do it this way? 

Therefore, let's start with an example,

Write a method that accepts two positive numbers as input and returns their sum

Let's try and do a mock TDD for the above problem. 

Case 1 - Function addPositive exists

  • it('should have addPositive method'); - The test case for this would fail at this point.

Therefore, writing code for it,

    function addPositive(a,b){}

Case 2 - Accepts two numbers as input

  • it('should have addPositive method'); - Passes now as we have the method written
  • it('should accept two numbers as input'); - The test case for this would fail because it does not throw any error
  • it('should not accept non numbers as input'); - The test case for this would fail because it does not throw any error

Therefore, writing code for the new test case to pass,

    function addPositive(a,b){
        if(typeof a !== "number" || typeof b !== "number"){
            throw new Error("Input arguments should be of type number");
        }
    }

Here, if you notice, we have written two cases. One for success scenario and another for failure scenario.

What is a failure scenario?

Failure scenario is nothing but a situation where we should get an error as expected for the invalid input. In our example, if we pass a non-number value or a negative value, we should expect to see an error thrown from the method. 

Case 3 - Accepts two positive numbers as input

  • it('should have addPositive method'); - Passes now as we have the method written
  • it('should accept two positive numbers as input'); - The test case for this would fail because it does not check if the numbers are positive.
  • it('should not accept negative numbers as input'); - The test case for this would fail because it does not check if the numbers are positive.

Therefore, updating the code for the updated test case to pass,

    function addPositive(a,b){
        if((typeof a !== "number" || typeof b !== "number") && (a < 0 || b < 0)){
            throw new Error("Input arguments should be of positive numbers");
        }
    }

One more thing to note here, since the cases are similar, we combine the test cases and update the existing cases instead of writing new one.

Case 4 - Returns sum of the numbers

  • it('should have addPositive method'); - Passes now as we have the method written
  • it('should return sum by accepting two positive numbers as input'); - The test case for this would fail because it does not return the sum yet.
  • it('should not accept negative numbers as input'); - The test case passes because our method now throws error when the input is either negative or if it is non-number.

Therefore, updating the code for the updated test case to pass,

    function addPositive(a,b){
        if((typeof a !== "number" || typeof b !== "number") && (a < 0 || b < 0)){
            throw new Error("Input arguments should be of positive numbers");
        }
        return a+b;
    }

Once again, we combine the test cases and update the existing case instead of writing new one.

With the above code, we have completed writing test cases and also the method in no time and the test cases,

  • it('should have addPositive method'); - Passes now as we have the method written
  • it('should return sum by accepting two positive numbers as input'); - Passes now because it accepts two positive numbers as input and returns the sum as expected
  • it('should not accept negative numbers as input'); - The test case passes because our method now throws error when the input is either negative or if it is non-number.

Happy ending right? There's just one thing left out to do. Just split the test cases as two different suites namely the success & failure as below,

  • describe('success'):
    • it('should have addPositive method'); 
    • it('should return sum by accepting two positive numbers as input'); 
  • describe('failure'):
    • it('should not accept negative numbers as input');
I am hoping to write more around these in the coming days. Stay tuned!

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