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First steps:

1. Clone project by running: git clone https://github.com/nejcrogelsek/react-project-template.git

2. Delete .git file inside cloned project.

3. Run: yarn run start:clean

4. Run: yarn run husky:postinstall to install husky git hook (automatically formats the code when pushing code to github)

5. Before running the app you need to serve mock data with: yarn run serve-json. If you go to http://localhost:4000/superheroes you will see the data.

Link to offical website.

1. react-query cache

The first time useQuery is fired for superheros key, isLoading flag is set to true and network request is send to fetch the data. When the request is completed it is cached used the query key (superheroes) and the fetchSuperHeroes function as the unique identifiers.

When we revisit the page react query will check if the data for this query exists in cache. If the data exists in the cache, the cached data is immediately returned without isLoading flag set to true.

React query knows that the server data might be updated and that the cache doesn't contain the latest data. So a background refetch is triggered for the same query and if the response is successful the displayed data is updated in the UI. As our data in this example is the same as the cached data, we don't see any differences. But you can change the db.json file and see the changes for yourself.

To indicate the background refetch state, react-query provides us with the flag called isFetching.

React query cache the data so we don't need to see the loading indicator every time we visit the page.

The default cache time is set to 5 mintes. We can change it with options cacheTime property

2. react-query stale time

Let's say the data doesn't change that often. We can set the stale time to 30 seconds and the user will see the cached query results or cached data for 30 seconds before refetching it again. The default state time is set to 0 seconds.

0 seconds is possibly the safest value you can set to stale time, which is also the default value.

3. react-query refetchOnMount

If set to true, the query will refetch on mount if the data is stale. If set to false, will disable additional instances of a query to trigger background refetches. If set to 'always', the query will always refetch on mount. If set to a function, the function will be executed with the latest data and query to compute the value Defaults to true.

Default value is set to true.

4. react-query refetchOnWindowFocus

If set to true, the query will refetch on window focus if the data is stale. If set to false, the query will not refetch on window focus. If set to 'always', the query will always refetch on window focus. If set to a function, the function will be executed with the latest data and query to compute the value. Defaults to true.

Default value is set to true.

5. react-query refetchInterval - Polling

Fetching data at regular intervals. For example: If you have component that shows real time price of different stocks, you might want to fetch the data every second so is in sync with UI.

For that react-query provides us with refetchInterval flag.

Default value is set to false.

Polling is paused when the window lose focus. To fix that issue we can set refetchIntervalInBackground property to true.

6. Homework (Solution on branch: feature/09-homework)

Combine polling with callbacks. Use the refetchInterval option to pull the api data every 3 seconds. Behind the scenes add a fourth superhero of your choice to the superheroes array in db.json.

a) Within the onSuccess callback check if the number of heroes is 4 and ifit is the case I want you to stop the polling.
b) Within the onError callback I want you to stop the polling.

Hint: Mantain state variable whose initial value is 3000. State variable will be assigned to refetchInterval configuration. In callbacks check for the response / errors and set the state variable to false.

7. react-query select - Data Transformation

For data tranformation react-query provides us with select flag.

select is function that receives onSuccess response and transform out data in the way we want it.

In our example we transformed data to be an array of just heroes names instead of whole response.

8. QueryById - 2 solutions

We can query by id in two ways (look commits under branch name: feature/12-queryById).

	a) Solution 1 or first commit: **QueryById with manuali passed ID**
		- We can manually pass id into fetch function: file `useSuperHeroData`
	b) Solution 2 or second commit: **QueryById with react-query automatic passed id**
		- React query automatically pass id into fetch function: file `useSuperHeroData`

9. react-query - Parallel queries

Sometimes a single component needs to call multiple APIs to fetch the necessary data. With react-query with simply call useQuery twice.

10. react-query - Dependent queries

Dependent query - is dependent on the results of another query.

11. react-query initialData - Initial query data

On react-query heroes page we have displayed different heroes. If we click on the hero, we see loading text for a moment. To get rid of that initial loading statement, we can set initialData in useSuperHeroData.tsx file. If we refresh detail page, the loading button will show because we don't have any initial data at the start, because we skipped the fetch all super heroes query.

12. react-query keepPreviousData - Paginated queries

If we add flag keepPreviousData into the useQuery options, when we paginate to next page, the data from the previous page is still displayed till the new data is successfully fetched.

13. react-query - Handling mutation response

Handling mutation response will save you additional network request.

14. react-query - Optimistic updates Updating the state before performing a mutation under the assumption that nothing can go wrong.

onMutate: is called before the mutation function is fired and is passed the same variables that mutation function would receive. a) Cancel any outgoing refetches so they don't override our optimistic update. b) Get a hold of the current query data before we make any update. This will help us role back in case the mutation fails. onError: is called if the mutation encounters an error. onSettled: is called if the mutation is either successfull or encounters an error. invalidateQueries will ensure client state is in sync with server state. The user will not notice a difference.

In the project directory, you can run:

Remove node_modules & yarn.lock & yarn-error.log files. Then it runs: yarn to install all packages & finally yarn start to run the app in the development mode.\

Open http://localhost:3000 to view it in the browser.

The page will reload if you make edits.
You will also see any lint errors in the console.

Runs the app in the development mode.
Open http://localhost:3000 to view it in the browser.

The page will reload if you make edits.
You will also see any lint errors in the console.

Launches the test runner in the interactive watch mode.
See the section about running tests for more information.

Builds the app for production to the build folder.
It correctly bundles React in production mode and optimizes the build for the best performance.

The build is minified and the filenames include the hashes.
Your app is ready to be deployed!

See the section about deployment for more information.

Note: this is a one-way operation. Once you eject, you can’t go back!

If you aren’t satisfied with the build tool and configuration choices, you can eject at any time. This command will remove the single build dependency from your project.

Instead, it will copy all the configuration files and the transitive dependencies (webpack, Babel, ESLint, etc) right into your project so you have full control over them. All of the commands except eject will still work, but they will point to the copied scripts so you can tweak them. At this point you’re on your own.

You don’t have to ever use eject. The curated feature set is suitable for small and middle deployments, and you shouldn’t feel obligated to use this feature. However we understand that this tool wouldn’t be useful if you couldn’t customize it when you are ready for it.

Generates new page with specified name.

Generates new component with specified name.

• yarn update:check - Check for any packages updates.
• yarn update:single packageName - Updates one package.
• yarn update - Updates all packages.

• yarn code:check - Check for any prettier errors.
• yarn code:format - Format code.
• yarn code:format:specific-file - Format specific file.

• yarn lint - Check for any eslint errors.
• yarn lint:fix - Fix some of the errors.

You can learn more in the Create React App documentation.

To learn React, check out the React documentation.