"this" in react higher order component - javascript

I want to move shared functionality from my react components to a higher order component like this:
function withListFunctions(WrappedComponent) {
return class extends React.Component {
constructor(props) {
super(props);
}
// my shared functionality
deleteItem() {
// Do something, then ...
this.setState({itemDeleted: true});
}
render() {
return (
<WrappedComponent
deleteItem={this.deleteItem}
/>
);
}
}
Using this kind of syntax requires to explicitely bind this in the constructor of the HOC:
this.deleteItem = this.deleteItem.bind(this);
.. but I wanted to bind the wrapped component instead. So what I tried in my wrapped component's constructor was
this.props.deleteItem = this.props.deleteItem.bind(this);
But this just resulted in a "Cannot assign to read only property" Error, as react props are meant to be read only.
I am aware that I could store the state item in the HOC and pass it down as a prop. But it would then be not accessible(writeable) anymore by other wrapped components functions, right? I wonder if there is a way to just share the unbound function and then bind it to the wrapped instance.
[Edit] I marked Abdul Rauf's answer as "accepted", nevertheless I want to state that Karen Grigoryan's answer is the solution I am actually using because it works and seems just appropiate for my app's complexity.

I am aware that I could store the state item in the HOC and pass it
down as a prop. But it would then be not accessible(writeable) anymore
by other wrapped components functions, right?
You should store shared state in HOC. You can pass multiple state update methods to Wrapped components that they can call internally to update state indirectly.
If you don't want to pass multiple state update methods then we have 2 options:
Option 1: Create a single dispatch method in HOC that takes action and optional payload and pass it to Wrapped components.
// Do not mutate state in reducer. always return a new state
reducer(state, action) {
switch (action.type) {
case 'delete':
// return final state after delete
case 'add':
// return final state after add using action.payload
case 'update':
// return final state after update using action.payload
default:
// A reducer must always return a valid state.
// Alternatively you can throw an error if an invalid action is dispatched.
return state;
}
}
dispatch(action) {
const updatedState = this.reducer(this.state, action)
this.setState(updatedState);
}
Option 2: Use useReducer hook if you can use latest react with hooks support.
I wonder if there is a way to just share the unbound function and then
bind it to the wrapped instance.
Technically you can do that (Check Karen Grigoryan's answer). But it is considered a bad practice because of many reasons. Few of those are:
Its against encapsulation principles. (State is in Child component and state update logic is in parent component). Parent component shouldn't know anything about state of child components.
Props can change over time but this won't be reflected automatically in derived Instance properties/fields

props are read only by design.
React elements are immutable. Once you create an element, you can’t change its children or attributes. An element is like a single frame in a movie: it represents the UI at a certain point in time.
So technically one of the ways to bind deleteItem to WrappedComponent context, is to just bind it in WrappedComponent constructor:
codesandbox example
this.deleteItem = this.props.deleteItem.bind(this);

Related

Which React lifecycle method is a good replacement for componentWillRecieveProps?

I have heard of getDerivedStateFromProps but it does not work how I want it to.
I have this code:
class App extends Component {
static getDerivedStateFromProps(props, state) {
console.log("Get derived state from props called");
return null;
}
render() {
return (
<button onClick={() => this.setState({})}>Change State</button>
);
}
}
Clicking the button calls getDerivedStateFromProps. This is not what I need. I want a function that is called only when new props are received. When internal state changes, this function should not be called.
I intend to use this in a modal form scenario. When the component receives an object as a prop, the component should translate this object into a form, then place that form into the state. If I use getDerivedStateFromProps, since it is called in every this.setState(), it would not reflect the changes in the form since whenever the user types, a this.setState() is fired, and instead of the changes being set into the state, the initial object is.
Which React lifecycle method should I use?
There are a couple of ways to approach this problem depending on different use cases.
If new props are added, those new props can be checked in the Child component and user defined functions can be called in case the new object exists.
getDerivedStateFromProps(props, state, prevProps) is a static method and this does not exist in the same, which means that you'll have to return a state based on various conditions. That is, let it work on default on most cases. On cases where you want to alter its functionality, place checks and return the modified state.
Hope it answers :)

Calling state changing functions from mapStateToProps

I am new to react-redux and I was surprised to see an example where a function, in this case being getVisiblieTodos, is called inside mapStateToProps. This function should be called in a reducer since it changes state? Is the code breaking "good form" for the sake of brevity? Is it okay to do this in general?
I am looking at code from this link
import { connect } from 'react-redux'
import { toggleTodo } from '../actions'
import TodoList from '../components/TodoList'
const getVisibleTodos = (todos, filter) => {
switch (filter) {
case 'SHOW_ALL':
return todos
case 'SHOW_COMPLETED':
return todos.filter(t => t.completed)
case 'SHOW_ACTIVE':
return todos.filter(t => !t.completed)
}
}
const mapStateToProps = (state) => {
return {
todos: getVisibleTodos(state.todos, state.visibilityFilter)
}
}
const mapDispatchToProps = (dispatch) => {
return {
onTodoClick: (id) => {
dispatch(toggleTodo(id))
}
}
}
const VisibleTodoList = connect(
mapStateToProps,
mapDispatchToProps
)(TodoList)
export default VisibleTodoList
In redux we want the store to hold the minimal data needed for the app. Everything that is derived from the base data, should be computed on the fly, to prevent cloning pieces of the store, and the need to recompute all derived data when something changes in the store.
Since the visible todos list is not part of the store, but computed using the list of todos, and the visibilityFilter, the getVisibleTodos() doesn't change the store's state. It produces the derived computed data from the those two properties.
A function that is used to get data from the store, and compute derived data is known as a selector. Using selectors, the derived data is not part of the store, and computed when needed. In addition, we can use memoized selectors, to save the computation overhead.
You may see getVisibleTodos as a reducer because it includes "switch .. case" block or/and because it has 2 arguments . However, it is not a rule.
A redux reducer ( by definition) changes store state according to dispatched action , and that's why it takes two arguments ( store state + dispatched action ) and it returns new state for the store without mutation.
getVisibleTodos here is a helper function which filter an array according to string (filter).
Also , filter is not a redux-action, it is just string that decides todos to be rendered.
I may agree with you it is something weird , and if we can see the whole application (reducers, actions,... ) we can decide if it is best practices or not .
todos in this component is a calculated property based on the state of the reducer, and it is not changing any state.
It's okay to transform properties comming from recuders that are used only by one component (they are called selectors). Imagine that you use todos in other components, you will not want to make changes in one component like filtering and seeing that in the other components. If this is the case, it's fine to do it.
Also, it is a good property of your reducer to store only the needed data. More state is more complexity in the app, and more overhead to calculate new states.
It seems to me that a function should do what its name says, nothing less, nothing more.
mapStateToProps() should just do that, ie "map", and should normally not call other functions.

Where to store values calculated from props in React

What is the best place to store the result of an expensive calculation from the React props which I use in render() but do not want to execute at each render() ?
constructor(props) {
super(props)
const result = this.doExpensiveCalculation(props)
}
componentWillReceiveProps(nextProps) {
// if nextProps differ from props
const result = this.doExpensiveCalculation(nextProps)
}
doExpensiveCalculation(props) {
// Some expensive stuff
}
render(){
// Use doExpensiveCalculation(this.props) here
}
The options are this and state but both I see rather unsatisfying. Is there a ready solution which uses memoisation?
On the other hand, should I worry about optimizing this ? I read that React can rerender component even if the props have not changed but does this happen often ?
You can handle the re-rendering in the lifecycle method of shouldComponentUpdate. Default value is always return true. By returning false there React will not re-render the component.
See the docs for more. Besides that, React only updates if a state change occurs since props are read-only.
Your options are to store it as you suggested or have a class with a static field to keep it there.
If all you want to do is perform the expensive calculation whenever you get new props, instead of on every render, you probably want componentWillReceiveProps:
componentWillReceiveProps() is invoked before a mounted component receives new props.
As far as where to store them, you can either store them in state, or as a property directly on the component instance. Either will work just as well.
You want to make sure compare values though, to avoid unnecessarily recomputing.
For example:
componentWillReceiveProps(nextProps) {
if (nextProps.someValue !== this.props.someValue) {
this.someResult = this.performExpensiveCalculation(nextProps.someValue);
}
}

How is state passed to selectors in a react-redux app?

I came across an example, where the mapStateToProps function is using memoization. I was just wondering how the "state" parameter is passed onto memoized selectors. After looking at the docs for reselect as well as redux, it seems that the mapStateToProps can return a function which accepts state as its argument, and the connect decorator might be the one passing the state to it but am not sure. Can someone please shed some light?
views/tracklist/index.js
const mapStateToProps = createSelector(
getBrowserMedia,
getPlayerIsPlaying,
getPlayerTrackId,
getCurrentTracklist,
getTracksForCurrentTracklist,
(media, isPlaying, playerTrackId, tracklist, tracks) => ({
displayLoadingIndicator: tracklist.isPending || tracklist.hasNextPage,
isMediaLarge: !!media.large,
isPlaying,
pause: audio.pause,
pauseInfiniteScroll: tracklist.isPending || !tracklist.hasNextPage,
play: audio.play,
selectedTrackId: playerTrackId,
tracklistId: tracklist.id,
tracks
})
);
export default connect(
mapStateToProps,
mapDispatchToProps
)(Tracklist);
core/tracklists/selectors.js
export function getCurrentTracklist(state) {
// console.log(state);
let tracklists = getTracklists(state);
return tracklists.get(tracklists.get('currentTracklistId'));
}
export const getTracksForCurrentTracklist = createSelector(
getCurrentPage,
getCurrentTrackIds,
getTracks,
(currentPage, trackIds, tracks) => {
return trackIds
.slice(0, currentPage * TRACKS_PER_PAGE)
.map(id => tracks.get(id));
}
);
Overview of how state is passed down to a selector when we use the Connect component from react-redux
What is a selector?
A selector extracts a subset of data from a source.
Let us think of the Redux store as our 'front end database'. For the purposeIn a database if you want to extract a subset of the total data you execute a query. In a similar fashion selectors are our queries to the Redux store.
In the simplest case, a selector could just return the state of the entire store.
The reselect docs give us three great reasons to use selectors
Selectors can compute derived data, allowing Redux to store the
minimal possible state.
Selectors are efficient. A selector is not
recomputed unless one of its arguments change.
Selectors are
composable. They can be used as input to other selectors.
What is a higher order component?
A higher-order component is a function that takes an existing component and returns a new component.
Connect is a higher order component that be given a selector
Taken from this brilliant gist which gives a good explanation of connect.
connect() is a function that injects Redux-related props into your
component.
Connect is a higher order component that makes our React component know about the Redux store. When we call connect we can pass mapStateToProps and mapDispatchToProps. These functions define the way in which our new component will be connected to the redux store.
We can give it access to state by passing a mapStateToProps function as an argument.
We can also bind action creators to store.dispatch through mapDispatchToProps. The advantage of this is that we don't need to pass down the entire store in order for a component to have access to store.dispatch so that the component can dispatch its own Redux actions.
The mapStateToProps function we pass to Connect is a selector
From the react-redux docs
The mapStateToProps function takes a single argument of the entire
Redux store’s state and returns an object to be passed as props. It is
often called a selector.
Think of the object that is returned by mapStateToProps as the result of our query to the Redux store. The resulting
The mapStateToProps function should normally return a plain object.
The result of calling mapStateToProps will normally be a plain object representing the data we extracted from the redux store.
The higher order Connect component allows us to extend the functionality of an existing component by merging in the data from this new object with the component's existing props.
Since selectors are just functions we can connect them to the Redux store using the connect component as well.
However in some cases we can return a function. Why would we do this?
By returing a function in mapStateToProps we can hijack the rendering cycle of components and optimise performance
In advanced scenarios where you need more control over the rendering
performance, mapStateToProps() can also return a function. In this
case, that function will be used as mapStateToProps() for a particular
component instance. This allows you to do per-instance memoization.
By passing the mapStateToProps function as an argument to our higher order component our connected component will be updated anytime the some state has changed in the Redux store.
If these updates happen very frequently or the state tree is large then the reselect library is useful as it allows us to use memoized selectors.
This fancy word means that results of selector calls are stored in case they need to be retrieved again.
So if mapStatesToProps returned a plain object instead of a function then whenever our store state changed then we would have new props for our component.???
Connecting selectors to the store
If you are using React Redux, you can call selectors as regular functions inside mapStateToProps():
import { getVisibleTodos } from '../selectors'
const mapStateToProps = (state) => {
return {
todos: getVisibleTodos(state)
}
}
Sharing Selectors Across Multiple Components
We can give reselect selectors props just like components when using the reselect library. This allows us to share selectors across multiple components.
Say we have multiple toDo lists each with their own Id. We would still use the same getVisibleTodos selector for each toDo list instance but just pass a different id as a prop.
However the issue with this is that createSelector only returns the cached value when its set of arguments is the same as its previous set of arguments.
The reselect docs point out that we can overcome this limitation by returning a function inside mapStateToProps:
In order to share a selector across multiple components and retain
memoization, each instance of the component
needs its own private copy of the selector.
If the mapStateToProps argument supplied to connect returns a function
instead of an object, it will be used to create an individual
mapStateToProps function for each instance of the container.
By returning a function inside mapStateToProps we can overcome this limitation and memoization will work correctly.
For a more detailed explanation see this
Is so simple: let's give you an example, I have a mapStateToProps like this:
function mapStateToProps(state) {
return {
categoryHistory: getCategoryHistory(state,'extended')
}
}
then I've create a selector like this:
export const getCategoryHistory = (state, type) => createSelector([getTaxonomy, selectedCategoryID], (categories, categoryID) => categories.getIn([type, categoryID]) || [])(state)
The solution is to call createSelector() passing the state as parameters:
createSelector()(state)
inside the selector you can use all the parameter you want to pass through.
In the cases you mentioned, mapStateToProps is a function which takes in state and returning object. When you passed mapStateToProps to connect, you passed a function which accepts state provided by connect as its argument.
createSelector creates a function which can take in state and returning object as well. Therefore you can assign it to mapStateToProps and pass it into connect.
In documentation, you'll normally find the following:
const mapStateToProps = (state) => {
whatever code
}
and
export default connect(mapStateToProps, mapDispatchToProps)(Component)
where mapStateToProps takes in state argument which is provided by connect.
However, one can let mapStateToProps to be a selector as followed:
const mapStateToProps = createSelector(
whatever code
)
This is because createSelector can take in a state as followed:
createSelector(whatever code)(state)
and return an object, just like what you find a mapStateToProps does in documentation.

When to use state and when props?

I am studying the principles of react.
According to some reviews, some people says is better to keep your component stateless, what does it mean?
But other people says, that if you need to update your component, then you should learn how to set your state to the proper state.
I saw this.props / this.setProps and this.state / this.setState and I am confuse with that.
Something I am trying to figure is, how can I update a component by itself and not from a parent component? should I use props or state in this case?
I already read some docs about props and state, what I don't have clear, is: when to use one or another ?
Props vs. state comes down to "who owns this data?"
If data is managed by one component, but another component needs access to that data, you'd pass the data from the one component to the other component via props.
If a component manages the data itself, it should use state and setState to manage it.
So the answer to
how can I update a component by itself and not from a parent component? should I use props or state in this case?
is to use state.
Props should be considered immutable and should never be changed via mutation. setProps is only useful on a top-level component and generally should not be used at all. If a component passes another component a property, and the first component wants the second to be able to change it, it should also pass it a function property that the second component can call to ask the first component to update its state. For example:
var ComponentA = React.createClass({
getInitialState: function() {
return { count: 0 };
},
render: function() {
return <Clicker count={this.state.count} incrementCount={this.increment} />;
},
increment: function() {
this.setState({count: this.state.count + 1});
}
});
// Notice that Clicker is stateless! It's only job is to
// (1) render its `count` prop, and (2) call its
// `incrementCount` prop when the button is clicked.
var Clicker = React.createClass({
render: function() {
// clicker knows nothing about *how* to update the count
// only that it got passed a function that will do it for it
return (
<div>
Count: {this.props.count}
<button onClick={this.props.incrementCount}>+1</button>
</div>
);
}
});
(Working example: https://jsbin.com/rakate/edit?html,js,output)
For and object-oriented programming analogy, think of a class/object: state would be the properties you put on the class; the class is free to update those as it sees fit. Props would be like arguments to methods; you should never mutate arguments passed to you.
Keeping a component "stateless" means that it doesn't have any state, and all its rendering is based on its props. Of course, there has to be state somewhere or else your app won't do anything! So this guideline is basically saying to keep as many components as possible stateless, and only manage the state in as few top-level components as possible.
Keeping components stateless makes them easier to understand, reuse, and test.
See A brief interlude: props vs state in the React docs for more information.
Use state when you know the variable value is going to affect the view. This is particularly critical in react, because whenever the state variable changes there is a rerender(though this is optimized with the virtual DOM, you should minimize it if you can), but not when a prop is changed (You can force this, but not really needed).
You can use props for holding all other variables, which you think can be passed into the component during the component creation.
If you have want to make a multi-select dropdown called MyDropdown for example
state = {
show: true,
selected:[],
suggestions:this.props.suggestionArr.filter((i)=>{
return this.state.suggestions.indexOf(i)<0;
})
}
props={
eventNamespace:'mydropdown',
prefix : 'm_',
suggestionArr:[],
onItemSelect:aCallbackFn
}
As you can see, the objects in the state variable are going to affect the view some way or the other.
The objects in the props are mostly objects which should remain the same throughout the component life cycle. So these objects can be callback functions, strings used to namespace events or other holders.
So if you do want to update the component by itself, you need to have to look into how componentWillRecieveProps ,componentWillUpdate, componentDidUpdate and componentShouldUpdate works. More or less, this depends on the requirement and you can use these lifecycle methods to ensure that the rendering is within the component and not in the parent.

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