Importance of TDD

Last post I wrote about a series renamer that I wrote in node using TDD. The other day I went to use the renamer on a bunch of files and found that it threw the following error:

TypeError: Cannot read property '1' of null
      at Object.getShowDetails (nameParser.js:25:47)

From looking at the show titles I was renaming I realised there was a filename that I had not taken into account namely one like this:

cool_show_107.mp4

This title means that it is episode 7 of series 1. Previously the only formats accounted for were when the season and episode were prefixed with an ‘s’ and ‘e’ respectively. Which brings me round to today’s topic. How important it is to build your software using a TDD approach and the advantages it gives you.

Upon seeing this error I could tell straight away from the stack trace that the error was in nameParser.js. So the first thing I did was write a test to reproduce this error.


describe('and the series and episode number are in 3 digit format', function(){
    it('should return the information for the series and episode correctly', function(){
        var result = nameParser.getShowDetails('cool_show_102');
        result.seriesNumber.must.be(1);
        result.episodeNumber.must.be(2);
    });
});

When I run the tests after adding this unit test I get the same error so I know I have reproduced the defect. Now I have a failing test that once I get passing will mean the defect will never come back. Also note how easy this was for me to track down the defect because the code is decoupled and already heavily tested.

Once the test is written all that is left is to get it to pass by adding another regex to the nameParser to detect 3 consecutive numbers. Once this was passing I ran the renamer again on the list of files and found a different error. Some of the files were in this format:

cool_show_S01E07_encoding_x264.mp4

This caused an issue because 3 consecutive numbers were being picked up by the nameParser and because that regex was running before the one looking for seasons and episodes prefixed with ‘S’ and ‘E’ it was incorrectly giving season 2 and episode 64 for the above filename.

So to fix again using TDD this is very simple. Add another test reproducing the defect:


describe('and the series and episode number are specified and a 3 digit number appears after them', function(){
    it('should return the information for the series and episode correctly', function(){
        var result = nameParser.getShowDetails('cool_show_S01E02_cool_x264');
        result.seriesNumber.must.be(1);
        result.episodeNumber.must.be(2);
    });
});

To fix all I had to do was simply rearrange the order the regex statements run in the nameParser. 39 tests now passing. I then ran the series renamer on the files in question and it worked perfectly.

The lesson here is that by building software using a TDD approach it forces you to write decoupled components each with their own job. Which means that it’s easy to isolate defects when they occur and write tests that reproduce the defects. Once the test is passing that defect cannot reoccur without failing a test.

If you want to checkout the full code for the series renamer you can clone it on github.

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