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Finding a bug after writing a test, an example

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In almost every Sprint, at work, I try to reserve some space for solving tech debt and doing some refactoring. A part of resolving this debt is making integration tests. And sometimes it happens that those tests find some unknown bug. That's what happened a few days ago. As usual, the names of the methods and variables are just an example.

The page I was writing the tests on is a simple one. Some text, three buttons, and nothing more. Yet, it has some hooks as well, defining the logic of those buttons (opening another URL, submitting the form, etc).

I started writing the tests in the usual way, using React Testing Library. Checking the rendering and the behavior of the page by clicking on the different buttons. When I was writing the test for one of the three buttons, however, it forced me to let me think: is this button always visible? Why?

{showButton && (
   <Button onClick={onApplyClick}>
      DO THING
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I checked the hooks and found out that it would render only when another property was true, or present. So why it was always there?

  showButton: !!secondaryProp,
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I followed the source of that property and I came across the point where its data was fetched from the (easy-peasy) store, and then it was set as an initial value of 0, instead of being undefined.

const initialState: IState = {
  secondaryProp: 0,
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The type of that property was just "number", instead of "number?". So it was always true.

export interface IState {
  secondaryProp: number
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To fix it, I changed the type and set the initial values as undefined.

Re-checking that property I saw that another value was dependent on it being present or not. So in another hook, I had to redefine the behavior in case the prop was falsy.

const newUrl = secondaryProp && `${FETCHED_URL}/prop=${secondaryProp}`

const onClick = () => {
    if (!newUrl) {
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So that's an interesting case of how making tests helps you reflect on what the behavior, and the logic of the methods and components, should be.

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