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Test driven development is the best way to write code.

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Test driven development means writing tests before writing code.

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In this lesson, you're going to learn about test driven developments.

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First, open up the following lesson by navigating to this path in your course resources.

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A bad design is one code is not modular code that is not modular, performs many tasks at once.

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Here is an example of a bad design.

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This is code that we wrote in the last section, and the reason why this code is bad is because the

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checkout method is performing four tasks at once, which makes it impossible to test.

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You can easily get bugs from this code.

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Just by looking at it, it's so hard to understand what's going on and trust me, if someone else were

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to look at this, they'll say, what the heck happened here?

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This code is not scalable.

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If someone decides to change anything, it can easily break.

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Test driven development is what helps you achieve a modular design.

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Test driven development simply means writing tests before writing code.

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If a code is easy to test, then it's good code.

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So the best way to achieve a modular design is to write tests before writing code.

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And there are two parts to test driven development.

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First, you have to identify meaningful test cases.

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Then you have to write a unit test for each test case first, you write a test that fails, then you

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write code to make the test pass, and then you refactor the code if necessary.

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Open up the folder for this lesson, there's a text file with a requirement we need to fill fill the

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check out process, calculates the receipt and the form of a subtotal tax and total.

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The first step is to identify meaningful test cases, think about the requirement and make a list of

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test cases we need to test if the subtotal equals the sum of each price.

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We also need to test if the tax equals 13 percent of the subtotal.

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And then we need to test, if it returns a total, that equals the subtotal plus tax.

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That's all for part one of test driven developments, we identified meaningful test cases and in part

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two, we're going to write a unit test for each test case.

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To recap, test driven development helps achieve a modular design.

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There are two parts to test driven development, and in this video we implemented part one, we identified

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meaningful test cases.

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And Part two is going to be to write a unit test for each test case.

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We're going to write a test that fails, write code to make the test pass and then refactor the code

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if necessary.