🧘 Things I think about when I write React code 🧘

react-philosophies

If react-philosophies helped you in some way, consider buying me a few cups of coffee ☕☕☕. This motivates me to create more React "stuff"! 🙂

Table of Contents

0. Introduction
1. The Bare Minimum
2. Design for happiness
3. Performance tips
4. Testing principles

🧘 0. Introduction

react-philosophies is:

• things I think about before I write React code.
• at the back of my mind whenever I review someone else's code or my own
• just guidelines and NOT rigid rules
• a living document and will evolve over time as my experience grows
• mostly techniques which are variations of basic refactoring methods, SOLID principles, and extreme programming ideas... just applied to React specifically 🙂

A lot of these things may feel like very basic and common-sense. But surprisingly, I've worked with large complex applications where these things are not taken into consideration. The examples I present here are based on code I have actually seen in production.

react-philosophies is inspired by various places I've stumbled upon at different points of my coding journey.

Most notably:

• Tim Peters: Zen of Python (Pep 20)
• Dev Cheney: Zen of Go
• Sandi Metz
• Kent C Dodds
• Martin Fowler
• Dan Abramov
• Bob Martin (Not saying I agree with his political views)
• wiki.c2.com
• sapegin/washingcode-book
• trekhleb/state-of-the-art-shitcode
• droogans/unmaintainable-code

🧘 1. The Bare Minimum

1.1 Recognize when the computer is smarter than you

1. Statically analyze your code with ESLint. Enable the rule-of-hooks and exhaustive-deps rule to catch React-specific errors.
2. Typescript will make your life so much easier.
3. NextJS is an awesome framework.
4. Be honest about your dependencies. Fix exhaustive-deps warnings / errors on your useMemo's, useCallback's and useEffect's. You can try "The latest ref pattern" to keep your callbacks always up-to-date without unnecessary rerenders.
5. Always add keys whenever you use map to display components.
6. Only call hooks at the top level. Don’t call Hooks inside loops, conditions, or nested functions.
7. Understand the warning "Can't perform state update on unmounted component". See PR: facebook/react/pull/22114, Reddit/u/free_username17
8. There is a reason why errors and warnings are displayed in the console.
9. Prettier (or an alternative) formats your code for you, giving you consistent formatting every time. You no longer need to think about it!
10. I highly recommend Code Climate (or similar) for open-source repositories or if you can afford it. I find that automatically-detected code smells truly motivates me to reduce the technical debts of the application I'm working on!
11. tree-shaking is your friend!
12. Prevent the "white screen of death" by adding several error boundaries at different levels of your application. You can also use them to send alerts to an error monitoring service such as Sentry if you want to.
13. Enable "strict" mode. It's 2021.

1.2 Code is just a necessary evil

"The best code is no code at all. Every new line of code you willingly bring into the world is code that has to be debugged, code that has to be read and understood, code that has to be supported." - Jeff Atwood

"One of my most productive days was throwing away 1000 lines of code." - Eric S. Raymond

"I hate code and I want as little of it as possible in our product" - Jack Diederich

"If I had more time, I would have written a shorter letter" - Blaise Pascal, Mark Twain, among others..

See also: Write Less Code - Richard Hariss (Svelte), Washing Code: Code is evil - Artem Sapegin

TL;DR

1. Think first before adding another dependency
2. Eliminate code with techniques not unique to React
3. Don't be clever. YAGNI!

1.2.1 Think first before adding another dependency

Needless to say, the more you add dependencies, the more code you ship to the browser. Ask yourself, are you actually using the features which make a particular library great?

1. Do you really need Redux? It's possible. But keep in mind that React is already a state management library.
2. Do you really need Apollo client ? Apollo client has many awesome features, like manual normalization. However, it will significantly increase your bundle size. If your application only makes use of features that are not unique to Apollo client , consider using a smaller library such as react-query or SWR (or none at all).
3. Axios? Axios is a great library with features that are not easily replicable with native fetch. But if the only reason for using Axios is that it has a better looking API, then consider just using a wrapper on top of fetch (such as redaxios or your own). Determine whether or not your application is actually using Axios's best features.
4. Lodash/underscoreJS? you-dont-need/You-Dont-Need-Lodash-Underscore
5. MomentJS? you-dont-need/You-Dont-Need-Momentjs
6. You might not need Context for theming (light/dark mode), consider using css variables instead.
7. You might not even need Javascript. CSS is powerful. you-dont-need/You-Dont-Need-JavaScript

1.2.2 Eliminate code with techniques not unique to React

React is just Javascript and Javascript is just code

1. Simplify complex conditionals and exit early if you can.
2. If there is no discernable performance difference, replace traditional loops with chained higher-order functions (map, filter, find, findIndex, some, etc)

1.2.3 Don't be clever. YAGNI!

"What could happen with my software in the future? Oh yeah, maybe this and that. Let’s implement all these things since we are working on this part anyway. That way it’s future-proof."

You Aren't Gonna Need It! Always implement things when you actually need them, never when you just foresee that you may need them.

See also: Martin Fowler: YAGNI, C2 Wiki: You Arent Gonna Need It!, C2: YAGNI (original), Jack Diederich: Stop Writing Classes

1.3 Detect code smells and do something about them if you need to

A window gets broken at an apartment building, but no one fixes it. It's left broken. Then something else gets broken. Maybe it's an accident, maybe not, but it isn't fixed either. Graffiti starts to appear. More and more damage accumulates. Very quickly you get an exponential ramp. The whole building decays. Tenants move out. Crime moves in. And you've lost the game. It's all over. You don't want to let technical debt get out of hand. You want to stop the small problems before they grow into big problems. - Don't Live with Broken Windows: A Conversation with Andy Hunt and Dave Thomas, Part I

If you recognize that something is wrong, fix it right there and there. But if it's not that easy to fix or you don't have time to fix it at that moment, add a comment FIXME or TODO and add a short explanation of the problem you've identified. Make sure everybody knows it is broken. It shows other people that you care and that they should also do the same when they encounter those kinds of things.

1.4 Just because it works, doesn't mean it is right

TIP: Remember that you may not need to put your state as a dependency because you can pass a callback function instead. You don't need to put setState (from useState) and dispatch (from useReducer) in your dependency array for hooks like useEffect and useCallback. ESLint will NOT complain because React guarantees their stability.

🙈 Example

❌ Not-so-good
const decrement = useCallback(() => setCount(count - 1), [setCount, count])
const decrement = useCallback(() => setCount(count - 1), [count])

✅ BETTER
const decrement = useCallback(() => setCount(count => (count - 1)), [])

TIP: If your useMemo or useCallback doesn't have a dependency, you might be using it wrong.

❌ Not-so-good
const MyComponent = () => {
 const functionToCall = useCallback(x: string => `Hello ${x}! I am actually doing more than this`,[])
 const iAmAConstant = useMemo(() => { return {x: 5, y: 2} }, [])
 /* I will use functionToCall and iAmAConstant */
}
       
✅ BETTER 
const I_AM_A_CONSTANT =  { x: 5, y: 2 }
 const functionToCall = (x: string => `Hello ${x}! I am actually doing more than this`)
 const MyComponent = () => {
  /* I will use functionToCall and I_AM_A_CONSTANT */
}

🧘 2. Design for happiness

"Any fool can write code that a computer can understand. Good programmers write code that humans can understand." - Martin Fowler

"The ratio of time spent reading versus writing is well over 10 to 1. We are constantly reading old code as part of the effort to write new code. So if you want to go fast, if you want to get done quickly, if you want your code to be easy to write, make it easy to read." ― Robert C. Martin

TL;DR

1. 💖 Avoid state management complexity by removing redundant states
2. 💖 Pass the banana, not the gorilla holding the banana and the entire jungle (prefer passing primitives as props)
3. 💖 Keep your components small and simple- the single responsibility principle!
4. 💖 Duplication is far cheaper than the wrong abstraction (avoid premature / inappropriate generalization)
5. Avoid prop drilling by using composition (KCD: Prop Drilling). Context is not the solution for every state sharing problem
6. Split giant useEffects to smaller independent ones (KCD: Myths about useEffect)
7. Extract logic to hooks and helper functions
8. To break a large component, it might be a good idea to have logical and presentational components (but not necessarily, use your best judgement)
9. Prefer having mostly primitives as dependencies to useCallback, useMemo, and useEffect
10. Do not put too many dependencies in useCallback, useMemo, and useEffect
11. For simplicity, instead of having many useStates, consider using useReducer if some values of your state rely on other values of your state and previous state

💖 2.1 Avoid state management complexity by removing redundant states

When you have redundant states, some states may fall out of sync; you may forget to update them given a complex sequence of interactions. Aside from avoiding synchronization bugs, you'd notice that it's also easier to reason about and require less code. See also: KCD: Don't Sync State. Derive It!, Tic-Tac-Toe

Note: For the following two examples, assume that the number of items to be fetched in always less than 100 (meaning you don't need to worry about optimization). If you're working with really large numbers of items, you can memoize the some computations with useMemo.

🙈 Example 1

You are tasked to display properties of each right triangle from a list

• the lengths of each of the three sides
• the perimeter
• the area

A list of two numbers {a: number, b: number}[] should be fetched from an API. The two numbers represent the two shorter sides of a right triangle.

const TriangleInfo = () => {
 const [triangleInfo, setTriangleInfo] = useTriangles<{a: number, b: number}>([])
 const [hypotenuses, setHypotenuses] = useState<number[]>([])
 const [perimeters, setPerimeters] = useState<number[]>([])
 const [areas, setAreas] = useState<number[]>([])

 useEffect(() => {
   fetchTriangles().then(r => {
     setTriangleInfo(r)
     setHypotenuses(r.map(t => computeHypotenuse(t.a, t.b))
     setArea(r.map(t => computeArea(t.a, t.b))
   })
 }, [])

 useEffect(() => {
   setHypotenuses(triangleInfo.map(t => computeHypotenuse(t.a, t.b))
   setArea(triangleInfo.map(t => computeArea(t.a, t.b))
 }, [triangleInfo])

 useEffect(() => {
   const p = triangleInfo((t, i) => {
     return computePerimeter(t.a, t.b, hypotenuse[i])
   })
 }, [triangleInfo, hypotenuses])

 /*** show info here ****/
}

const TriangleInfo = () => {
  const [triangleInfo, setTriangleInfo] = useTriangles<{
    a: number;
    b: number;
  }>([]);

  useEffect(() => {
    fetchTriangles().then((r) => setTriangleInfo(r));
  }, []);

  const areas = triangleInfo.map((t) => computeArea(t.a, t.b));
  const hypotenuses = triangleInfo.map((t) => computeHypotenuse(t.a, t.b));
  const perimeters = triangleInfo.map((t, i) =>
    computePerimeters(t.a, t.b, hypotenuses[i])
  );

  /*** show info here ****/
};

🙈 Example 2

Suppose you are assigned to design a component which:

1. Fetches a list of unique points from an API
2. Includes a button to either sort by x or y (ascending order)
3. Includes a button to change the maxDistance (increase + 10)
4. Only displays the points that are NOT farther than the current maxDistance from the origin (0, 0)

type SortBy = 'x' | 'y'
const toggle = (current: SortBy): SortBy => current === 'x' ? : 'y' : 'x' 
const Points = () => {
  const [points, setPoints] = useState<{x: number, y: number}[]>([])
  const [filteredPoints, setFilteredPoints] = useState<{x: number, y: number}[]>([])
  const [sortedPoints, setSortedPoints] = useState<{x: number, y: number}[]>([])
  const [maxDistance, setMaxDistance] = useState<number>(Infinity)
  const [sortBy, setSortBy] = useState<SortBy>('x')
  
  useEffect(() => {
    fetchPoints().then(r => setPoints(r))
  }, [])
  
  useEffect(() => {
    setSortedPoints(sortPoints(points, sortBy))
  }, [sortBy, points])

  useEffect(() => {
    setFilteredPoints(sortedPoints.filter(p => getDistance(p.x, p.y) < maxDistance))
  }, [sortedPoints, maxDistance])

  const otherSortBy = toggle(sortBy)

  return (
    <>
      <button onClick={() => setSortBy(otherSortBy)}>Sort by: {otherSortBy}<button>
      <button onClick={() => setMaxDistance(maxDistance + 5)}>Increase max distance<button>
      Showing only points that are less than {maxDistance} units away from origin (0, 0)
      Currently sorted by: '{sortBy}' (ascending)
      <ol>{filteredPoints.map(p => <li key={`${p.x}|{p.y}`}>({p.x}, {p.y})</li>}
    </>
  )
}

// NOTE:
// You can also use useReducer instead
type SortBy = 'x' | 'y'
const toggle = (current: SortBy): SortBy => current === 'x' ? : 'y' : 'x'
const Points = () => {
  const [points, setPoints] = useState<{x: number, y: number}[]>([])
  const [maxDistance, setMaxDistance] = useState<number>(Infinity)
  const [sortBy, setSortBy] = useState<SortBy>('x')

  useEffect(() => {
    fetchPoints().then(r => setPoints(r))
  }, [])
  

  const otherSortBy = toggle(sortBy)
  return (
    <>
      <button onClick={() => setSortBy(otherSortBy)}>Sort by: {otherSortBy} <button>
      <button onClick={() => setMaxDistance(maxDistance + 10)}>Increase max distance<button>
      Showing only points that are less than {maxDistance} units away from origin (0, 0)
      Currently sorted by: '{sortBy}' (ascending)
      <ol>{
        sortPoints(
          points.filter(p => getDistance(p.x, p.y) < maxDistance),
          sortBy
        ).map(p => <li key={`${p.x}|{p.y}`}>({p.x}, {p.y})</li>
      }
    </>
  )
}

💖 2.2 Pass the banana, not the gorilla holding the banana and the entire jungle

You wanted a banana but what you got was a gorilla holding the banana and the entire jungle. - Joe Armstrong, creator of Erlang

To avoid falling into this trap, it's a good idea to pass mostly primitives (boolean, string, number, etc) types as props. (Passing primitives is also a good idea if you want to use React.memo for optimization)

A component should just know enough to do its job and nothing more. As much as possible, components should be able to collaborate with others without knowing what they are and what they do.

When we do this, the components will be loosely coupled. Loose coupling means that the degree of dependency between two components is low. Loose coupling makes it easier to change, replace, or remove components without affecting other components. See also: stackoverflow:2832017

🙈 Example

Create a UserCard component that displays two components: Summary and SeeMore. The SeeMore component includes presenting the age and bio of the user. Include a button to toggle between showing and hiding the age and bio of the user.

The Summary component displays the profile picture of the user and also his title, firstName and lastName (e.g. Mr. Vincenzo Cassano). Clicking the user's name should take you to the user's personal site. The Summary component may also have other functionalities. (Say for example, randomly changing the font, size of the image, and background color whenever this component is clicked.. for brevity let's call this "the random styling feature")

The UserCard calls the hook useUser that returns an object with the type below.

type User = {
  firstName: string;
  lastName: string;
  title: string;
  imgUrl: string;
  webUrl: string;
  age: number;
  bio: string;
  /****** 100 or more fields ******/
};

const Summary = ({ user } : { user: User }) => {
  /*** include "the random styling feature" ***/
  return (
    <>
      <img src={user.imgUrl} />
      <a href={user.webUrl}>{user.title}. {user.firstName} {user.lastName}</a>
    </>
  )
}

const SeeMore = ({ user }: { user: User }) => {
  const [seeMore, setSeeMore] = useState<boolean>(false)
  return (
    <>
      <button onClick={() => setSeeMore(!seeMore)}>See more</button>
      {seeMore && <>AGE: {user.age} | BIO: {user.bio}</>}
    </>
  )
}

const UserCard = () => {
  const user = useUser()
  return <><Summary user={user} /><SeeMore user={user} /></>
}

const Summary = ({ imgUrl, webUrl, displayName }: { imgUrl: string, webUrl: string, displayName: string }) => {
  /*** include "the random styling feature" ***/
  return (
    <>
      <img src={imgUrl} />
      <a href={webUrl}>{displayName}</a>
    </>
  )
}

const SeeMore = ({ componentToShow }: {componentToShow: ReactNode }) => {
  const [seeMore, setSeeMore] = useState<boolean>(false)
  return (
    <>
      <button onClick={() => setSeeMore(!seeMore)}>See more</button>
      {seeMore && <>{componentToShow}</>}
    </>
  )
}


const UserCard = () => {
  const { title, firstName, lastName, webUrl, imgUrl, age, bio } = useUser()
  return (
    <>
      <Summary displayName={`${title}. ${firstName} ${lastName}`} {...{imgUrl, webUrl}} />
      <SeeMore componentToShow={<>AGE: {age} | BIO: {bio}</>} />
    </>
  )
}

💖 2.3 Keep your components small and simple

(The paragraphs below is based on my 2015 article: Three things I learned from Sandi Metz’s book as a non-Ruby programmer)

What is the single responsibility principle?

A component should have one and only one job. It should do the smallest possible useful thing. It only has responsibilities that fulfill its purpose.

A component with various responsibilities is difficult to reuse. If you want to reuse some but not all of a its behavior, it's almost always impossible to just get what you need. It is also likely to be entangled with other code. Components that do one thing which isolate that thing from the rest of your application allows change without consequence and reuse without duplication.

How to know if your component has a single responsibility?

Try to describe that component in one sentence. If it is only responsible for one thing then it should be simple to describe. If it uses the word ‘and’ or ‘or’ then it is likely that your component fails this test.

Inspect the component's states, the props and hooks it consumes, as well as variables and methods declared inside the component (They shouldn't be too many). Ask yourself: Do these things actually work together to fulfill the component's purpose? If some of them don't, consider moving those somewhere else or breaking down your big component to smaller ones.

🙈 Example

The requirement is to display special kinds of buttons you can click to shop for items of a specific category. For example, the user can select bags, chairs, and food.

• Each button opens a modal you can use to select and "save" items
• If the user has "saved" selected items in a specific category, that category said to be "booked"
• If it is booked, the button will have a checkmark
• You should be able to edit your booking (add or delete items) even if that category is booked
• If the user is hovering the button it should also display WavingHand component
• A button should also be disabled when no items for that specific category is available
• When a user hovers a disabled button, a tooltip should show "Not Available"
• If the category has no items available, the button's background should be grey
• If the category is booked, the button's background should be green
• If the category has available items and is not booked, the button's background should be red
• For each category, it's corresponding button has a unique label and icon

type ShopCategoryTileProps = {
  isBooked: boolean
  icon: ReactNode
  label: string
  componentInsideModal?: ReactNode
  items?: {name: string, quantity: number}[]
}

const ShopCategoryTile = ({
  icon,
  label,
  items
  componentInsideModal,
}: ShopCategoryTileProps ) => {
  const [openDialog, setOpenDialog] = useState(false)
  const [hover, setHover] = useState(false)
  const disabled = !items || items.length  === 0
  return (
    <>
      <Tooltip title="Not Available" show={disabled}>
        <StyledButton
          className={disabled ? "grey" : isBooked ? "green" : "red" }
          disabled={disabled}
          onClick={() => disabled ? null : setOpenDialog(true) }
          onMouseEnter={() => disabled ? null : setHover(true)}
          onMouseLeave={() => disabled ? null : setHover(false)}
        >
          {icon}
          <StyledLabel>{label}<StyledLabel/>
          {!disabled && isBooked && <FaCheckCircle/>}
          {!disabled && hover && <WavingHand />}
        </StyledButton>
      </Tooltip>
      {componentInsideModal &&
        <Dialog open={openDialog} onClose={() => setOpenDialog(false)}>
          {componentInsideModal}
        </Dialog>
      }
    </>
  )
}

// split into two smaller components!
const DisabledShopCategoryTile = ({ icon, label }: { icon: ReactNode, label: string }) => {
  return (
    <Tooltip title="Not available">
      <StyledButton disabled={true} className="grey">
        {icon} <StyledLabel>{label}<StyledLabel/>
      </Button>
    </Tooltip>
  )
}

type ShopCategoryTileProps = {
  isBooked: boolean
  icon: ReactNode
  label: string
  componentInsideModal: ReactNode
}

const ShopCategoryTile = ({
  icon,
  label,
  isBooked,
  componentInsideModal,
}: ShopCategoryTileProps ) => {
  const [openDialog, setOpenDialog] = useState(false)
  const [hover, setHover] = useState(false)

  return (
    <>
      <StyledButton
        disabled={false}
        className={isBooked ? "green" : "red"}
        onClick={() => setOpenDialog(true) }
        onMouseEnter={() => setHover(true)}
        onMouseLeave={() => setHover(false)}
      >
        {icon}
        <StyledLabel>{label}<StyledLabel/>
        {isBooked && <FaCheckCircle/>}
        {hover && <WavingHand />}
      </StyledButton>
      <Dialog open={openDialog} onClose={() => setOpenDialog(false)}>
        {componentInsideModal}
      </Dialog>
    </>
  )
}

Note: This is a simplified version of a component that I've actually seen in production

const ShopCategoryTile = ({
  item,
  offers,
}: {
  item: ItemMap;
  offers?: Offer;
}) => {
  const dispatch = useDispatch();
  const location = useLocation();
  const history = useHistory();
  const { items } = useContext(OrderingFormContext);
  const [openDialog, setOpenDialog] = useState(false);
  const [hover, setHover] = useState(false);
  const isBooked =
    !item.disabled && !!items?.some((a: Item) => a.itemGroup === item.group);
  const isDisabled = item.disabled || !offers;
  const RenderComponent = item.component;

  useEffect(() => {
    if (openDialog && !location.pathname.includes("item")) {
      setOpenDialog(false);
    }
  }, [location.pathname]);
  const handleClose = useCallback(() => {
    setOpenDialog(false);
    history.goBack();
  }, []);

  return (
    <GridStyled
      xs={6}
      sm={3}
      md={2}
      item
      booked={isBooked}
      disabled={isDisabled}
    >
      <Tooltip
        title="Not available"
        placement="top"
        disableFocusListener={!isDisabled}
        disableHoverListener={!isDisabled}
        disableTouchListener={!isDisabled}
      >
        <PaperStyled
          disabled={isDisabled}
          elevation={isDisabled ? 0 : hover ? 6 : 2}
        >
          <Wrapper
            onClick={() => {
              if (isDisabled) {
                return;
              }
              dispatch(push(ORDER__PATH));
              setOpenDialog(true);
            }}
            disabled={isDisabled}
            onMouseEnter={() => !isDisabled && setHover(true)}
            onMouseLeave={() => !isDisabled && setHover(false)}
          >
            {item.icon}
            <Typography variant="button">{item.label}</Typography>
            <CheckIconWrapper>
              {isBooked && <FaCheckCircle size="26" />}
            </CheckIconWrapper>
          </Wrapper>
        </PaperStyled>
      </Tooltip>
      <Dialog fullScreen open={openDialog} onClose={handleClose}>
        {RenderComponent && (
          <RenderComponent item={item} offer={offers} onClose={handleClose} />
        )}
      </Dialog>
    </GridStyled>
  );
};

💖 2.4 Duplication is far cheaper than the wrong abstraction

Avoid premature / inappropriate generalization. If your implementation for a simple feature requires a huge overhead, consider other options. Sandi Metz's article on this is a must read! Sandi Metz: The Wrong Abstraction.

See also: KCD: AHA Programming, C2 Wiki: Contrived Interfaces, C2 Wiki: The Expensive Setup Smell, C2 Wiki: Premature Generalization

🧘 3. Performance tips

Premature optimization is the root of all evil - Tony Hoare (popularized by Donald Knuth)

TL;DR

1. If you think it’s slow, prove it with a benchmark. "In the face of ambiguity, refuse the temptation to guess."
2. Know the terms lazy loading and bundle/code splitting
3. Use useMemo mostly just for expensive calculations
4. For React.memo, useMemo, useCallback for reducing re-renders, they shouldn't have many dependencies and the dependencies should be mostly primitive types
5. Make sure your React.memo, useCallback and useMemo is doing what you think it's doing (is it really preventing rerendering?)
6. Window large lists (with tannerlinsley/react-virtual or similar)
7. Put Context as low as possible in your component tree. Context does not have to be global to your whole app
8. Context should be logically separated, do not add to many values in one context provider
9. You can optimize context by separating the state and the dispatch function
10. Stop punching yourself everytime you blink (fix slow renders before fixing rerenders)
11. Putting your state as close as possible to where it's being used will make your app faster
12. If you're going to use a package for your forms I recommend react-hook-forms. I think it is a great balance of good performance and good developer experience.
13. You can visualize the code bundles you've generated with tools such as source-map-explorer or @next/bundle-analyzer (for NextJS).

🧘 4. Testing principles

Write tests. Not too many. Mostly integration. - Guillermo Rauch, creator of Socket.io (and other awesome things)

TL;DR

1. Your tests should always resemble the way your software is used
2. Stop testing implementation details
3. If your tests don't make you confident that you didn't break anything, then they didn't do their (one and only) job
4. For the front-end, you don't need 100% code coverage, about 70% is okay
5. You should very rarely have to change tests when you refactor code
6. I like using Jest, React testing library, Cypress, and Mock service worker