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React > Redux > Flashcards

Redux Flashcards

1
Q

When redux is useful? What are the other options?

A

Redux is useful to share data with different components.

props (lifting state), react context and redux

props is the simplest and redux the more powerful and complex.

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2
Q

What are the 3 principles of Redux?

A

1 - One immutable store;
2 - Actions triggers changes;
3 - Reducers update state;

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3
Q

What a Reducer consists of?

A

a method containing the current state, an action to change the state and it returns a new state

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4
Q

What is the philosophy of redux?

A

data goes down and actions go up

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5
Q

What is a Reducer?

A

A pure function

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6
Q

What are the three common ways to do immutable updates?

A

object.assign, spread operator and map/filter/reduce.

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7
Q

What is a common pitfall of immutability of nested objects? Is that always an issue?

A

The clone is shallow and won’t clone inner objects. Not always an issue because you only need to clone stuff you’ll actually change.

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8
Q

What is the danger in the clone-deep?

A

It clones everything and is likely to cause excessive re-renders.

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9
Q

What is the most common lib to have immutability out-of-the-shelf?

A

Immer

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10
Q

What is an action? What property the action must have?

A

Represents user intent. Must have a type. Can have anything else extra to the type…

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11
Q

When an action is dispatched, how many reducers will be notified?

A

All of them, always.

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12
Q

From redux perspective, what are the differences between container and presentation components?

A

in simple terms: only containers know redux

detailed here: https://pasteboard.co/JmfOcHE.png

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13
Q

When reselect is a good choice? what does it do?

A

when selectors (mapStateToProp) do expensive operations. it does memoization

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14
Q

What are the two most common ways to mapDispatchToProps?

A

using bindActionCreators or returning an object

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15
Q

Why binding in the constructor is better than binding directly in the onChange assignment?

A

Because binding in the assignment will cause a new function to be allocated everytime the component renders.

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16
Q

What is a class field and what does it do the bindings?

A

A class field is an arrow function that receives an event as argument. It removes the need of bindings.

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17
Q

Why an arrow function removes the need of bindings?

A

Because “Arrow functions inherit the binding context of their enclosing scope”. Therefore, the this keyword is actually referring to the scope - as you might expect.

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18
Q

What does a class field do the initialization of the state of a class? what is the main benefit?

A

it removes the need of defining constructor and also removes the need of “this.” keyword… less code!

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19
Q

Why react hooks is so attractive?

A

Because it pretty much removes the need of creating class components.

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20
Q

Why is it recommended to define the onSubmit event in the form instead of handling a button click?

A

For accessibility and usability reasons: the user will be able to hit enter and submit the form

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21
Q

What is an action creator? Can it be a anonymous object?

A

Is a function that creates an action object (remember, must have a type). Yes, as usual people use anonymous in js for everything.

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22
Q

Why is it important to always return a default in the reducer “type” switch?

A

Because all reducers are called, so if your reducer don’t care about the message that was dispatched, then it must return the same state it received.

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23
Q

What is normalizing state shape? where to find more information?

A

State shape is how the state object is defined… having it as id: 1 {id: 1, name:”asd”} helps during find operations. See normalizing state shape in redux website.

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24
Q

Can i use something else other than a switch in the reducers to filter out the action type?

A

Yes, anything you want.

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25
Q

Does the name of the import must match the export default function/type?

A

No. Once it is an export default, you can name it anything you want.

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26
Q

Besides the initial state and the root reducer, what else can be applied in the CreateStore function?

A

You can applyMiddleware

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27
Q

What is a common and recommended middleware to be applied in the redux store? Why?

A

reduxImmutableStateInvariant. So that it will warn you when someone mutate the store’s state.

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28
Q

What is redux dev tools? What is the necessary import from redux to make it work?

A

Is a tool that allows playing with the application’s state directly in the browser. Necessary to import “copose” from redux.

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29
Q

When is it a good idea to configure the store with a different initial state different than the default empty?

A

Seems to be more useful for server side to do some sort of re-hydrate.

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30
Q

Where is a good place to put the Provider? Which lib does it come from? What param does the Provider need?

A

The (declarative) Provider must wrap the Router and the App. Comes from the react-redux lib. The provider needs the ConfigureStore as param.

31
Q

What to do with the warning dispatch is missing in the props validation?

A

need to import propTypes from “prop-types” and then declare that dispatch is required.

32
Q

How to connect a component to redux? Is the mapDispatchToProps required?

A

By decorating the export of the component with the Connect function. mapDispatchToProps is not required and will be auto injected by the lib.

33
Q

What dos the bindActionCreator do internally?

A

It wrapps the method passed as the first argument so that it mimics its call and then forward to the dispatcher (second argument).

34
Q

How to store the strings of each action?

A

as constant in a new file inside the action folder

35
Q

Why mocking api is a good idea? How to simulate slow APIs? Which is a lib to mock the API?

A

Speeds development. By using setTimeout. jsonServer

36
Q

Is fetch builtin in the browsers? also legacy ones?

A

fetch is builtin in modern browsers.

37
Q

Where to store environment variables?

A

in webpack.config.dev.js… any webpack config used

38
Q

what is run-p in npm? when is it useful?

A

run parallel… useful to start more than one project at once.. e.g: front and backend.

39
Q

What does Redux Middleware do?

A

Allow us to insert our own logic after an action is dispatched and before it is processed by the reducer.

40
Q

What are 4 common usages of Redux Middleware?

A

handling async api calls
logging
crash reporting
routing

41
Q

Should we always create our own middleware? What is the most common lib to handle asyncs?

A

No… there are several libs available. redux-thunk

42
Q

What is a Thunk? What is its main benefit?

A

a function that returns a function. Components won’t be aware on how action creators are implemented, neither if are sync or async.

Keep the code simple, widely used.

43
Q

What are the three main reasons of using async middleware?

A

Consistency;
Purity;
Easier Testing

44
Q

What to do when the this.props becomes very repetitive in the source code?

A

it is recommended to {desestructure, all, the, props} = this.props

45
Q

How to make the componentDidMount not fetch data from the API all the time?

A

By checking the length of the data arrays before dispatching the action.

46
Q

When the history object is available to a react component? Where to get it?

A

When the component is loaded by the element. Available in the props.

47
Q

What is an optimistic update?

A

Is when we update the UI immediately after the user does the action, therefore not waiting for the success return of an api call.

48
Q

Are fragments preferable over divs to tie components together?

A

Yes. So we avoid needless elements in the DOM.

49
Q

How to create a toast in react?

A

By using react-toastify lib

50
Q

What does async/await use behind the scenes on JS? Why using it then?

A

It uses promises. Make more clear when a method is actually async.

51
Q

Why using Enzyme for testing? What Enzyme runs behind the scenes? What does it uses to render the DOM?

A

Enzyme is very user friendly. Uses React Test Utils. Uses JSDOM to render the DOM.

52
Q

What is Jest? Is it a lib?

A

Jest is the most famous js testing FRAMEWORK.

53
Q

How does Jest finds the tests if it only requires “test”: “jest” config in the packages.json?

A

jest automatically finds test in files that end in .test.js or spec.js.

54
Q

Does jest need any other config than packages.json?

A

Yes… add mock for file and style. Also add ignored file extensions

55
Q

What is a test snapshot? What is the value behind it?

A

A snapshot is a pre-rendered component in a given state (usually defined in the testcase itself) which protect us from causing undully changes to the component output.

56
Q

What is the differente between shallow and mount on Enzyme?

A

shallow renders a single component and mount renders the component with children.

57
Q

What is the main Enzyme function to select HTML elements to test?

A

find

58
Q

What is a good trick to make the unit tests easier?

A

By creating a component factory method with default props and allow defining props externally as well.

59
Q

How to print the debug information of a JSX component (rendered HTML)? Will it show the assigned values in the props?

A

console.log(form.debug()). Yes, will show the assigned values in the props as well.

60
Q

What is the main difference of shallow and mount to keep in mind when testing?

A

Shallow will test JSX only (which is pretty much text) and mount will contain a !!RENDERED DOM!!.

61
Q

What are the main pro of shallow and mount?

A

shallow is fast and lightweight. Mount is slower but more realistic.

62
Q

When should I use mount?

A

When you want to test the dom or test interactions with the child components.

63
Q

What is a common alternative of Enzyme?

A

React testing library

64
Q

What is the philosophy of react testing library?

A

Write tests about what the users should see. Encouraging writting accessible components.

65
Q

Does react testing library contain a “shallow” form of rendering?

A

No. It always do a “mount”, due to its philosophy.

66
Q

What two main things when testing Redux components?

A

The markup itself and the behaviors

67
Q

Should I always wrap routed components in a MemoryRouter?

A

No, you might only pass down the necessary props such as history: {}

68
Q

Do I need to wrap a redux component in a provider along with a store to test it? are there other options?

A

Yes… it will show a very informative error in the console log. Exporting the component without the connect wrapper is a prefered option to keep the test simple.

69
Q

What is the hard part of testing Thunks? How to overcome?

A

Because it depends on the Store and make HTTP calls. Can be overcame by using redux-mock-store and fetch-mock.

70
Q

What kind of test we use to test the redux Store? which components does it involve? Does the course author like it? why?

A

integration tests. involves the integration between the action, store and reducers. he does like it because it covers a lot of code with few lines of code.

71
Q

Does the webpack generate physical files for development builds? What are these files?

A

NO. Only for production builds:
/build/index.html
/build/bundle.js
/build/styles.css

72
Q

What are the 8 items a production builds requires?

A 
lint and run tests;
bundle and minify JS and CSS;
Generate JS and CSS sourcemaps;
Exclude dev-specific concers;
Build React in production mode;
Generate bundle report;
Run the build on a webserver
73
Q

What is rimraf?

A

crossplatform package to delete files

74
Q

What else to configure other than the store and the packages.json?

A

Necessary to configure the webserver to redirect all the requests to the index.html. this is a SPA thing

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