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What is testing? As I said, testing does not mean that we test our application manually,

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we obviously do that and we should do that. With testing here,

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I mean that we write automated tests.

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So what we typically do is we build an application,

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then of course we test it manually in the browser and then we ship it to a server,

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now there's one extra step we can add in any development workflow and that is that we don't just ship

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the app after testing it manually but that we also have automated tests.

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Now these are tests which run automatically, we write them and then each test tests a tiny fraction of

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our application,

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that's a so-called unit test, 

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it tests a unit of our app.

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The idea behind such tests is that since we define these tests,

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if we change anything in our application and that breaks our app or breaks a component in our app then

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the respective test will fail and hence we might get a warning about a potential error we introduced

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before we even have to find it by manually testing the browser. In more complex applications,

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this is especially important since there, it's way easier to break something without quickly noticing

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it. There also is a different way of thinking about testing,

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instead of adding it in the end of the build workflow, there also is this idea of test driven development

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which you might have heard about, test driven development imports the tests first, there you write the

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tests before you write any application code

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and therefore all the tests will of course fail initially but then you add the application code and

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the test should pass step by step.

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That's not the concept

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I will show you here,

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this is no complete testing course, not at all

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instead I want to show you how to write these unit tests and you can of course then build up on this

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knowledge to use it in whichever way you want to use it.

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This however is the core idea of what testing is and why would we test?

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Well that is exactly what I already mentioned,

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we might have an application and there, we might have a component which does a couple of things as suggested

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here on the left and in the first draft we created for the component,

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it may pay us all the tests we defined for this given component

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so we wrote test for a given component and they pass.

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Now when we change something in this application or we add a new feature and that affects this component

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and we change something in that component for that reason then we still might have some passing tests

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but maybe some of our tests then fail

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and that's exactly showing us then where we need to look into our code and potentially fix or adjust it

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or fix or adjust our test to the changed requirements, either of the two,

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but at least we have to take a look at that

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so that's the idea behind testing. Now to be able to test,

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we need some testing tools.

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Let's take a closer look at these in the next lecture.