How bad is it to change the DOM in react? - javascript

I'm a noob react developer, and I'm currently building a MERN App.
My question to the community is, in my project I had to modify the DOM multiple times as shown below:
document.getElementsByTagName('body')[0].style = 'overflow: hidden';
I know that changing the DOM very often is not recommended in React.Js. So is it ok if I did it only to cut the body's scroll-bar?

In React, changing the DOM directly is usually bad because the state of the page should come directly from the state in React.
But React will render inside the body - the body can't be something returned by the JSX inside a React component, so doing
document.body.style = // ..
really is the only way to change the body's style.
The time you wouldn't want to do such a thing would be if the element being changed was being rendered by React, eg:
// some functional component
const clickHandler = () => {
document.querySelector('.foo').style.backgroundColor = 'green';
};
return (
<div className="foo" onClick={clickHandler}>foo</div>
);
because you could instead toggle some state inside the component which changes the returned JSX to include the different style.
That said, your approach of
document.getElementsByTagName('body')[0].style = 'overflow: hidden';
should be reconsidered, if possible:
Use document.body instead of getElementsByTagName
Do you really have to set the style of the whole <body>? It would be more inline with React to set the style of an element React is rendering, if possible - and then you can use the method mentioned above, of using state and the returned JSX instead of using DOM methods.

Related

How to access dom element that is not inside JSX in React?

We are using ReactJS framework, so we don't find element written in the entire code. The only way is to find the DOM element and should set the attribute. I am doing this in app.js.
setTimeout(function () {
const formElement = document.getElementsByTagName('form')
formElement.item(0).setAttribute('autocomplete', 'off')
}, 1000)
The above solution is working fine, but if the page loading is slow, then this will not set the attribute. Is there any other way to find the element and set the attribute? I have also tried to set it using the below code after DOMContentLoaded, but it's not working. Nothing inside this event is working.
document.addEventListener('DOMContentLoaded', () => {
const formElement = document.getElementsByTagName('form')
formElement.item(0).setAttribute('autocomplete', 'off')
})
You don't find the element because 'DOMContentLoaded' gets fired when HTML file completes loading, not when React renders the form.
How about using MutationObserver instead? https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver
Also doing that in root level app.js sounds wrong. Can you do it in the component that is closest to the container of the form?
In React a good way to access DOM elements is by using useRef hook. However, if that element you want to query is not in the JSX of your current component, the useEffect hook could be used with normal JavaScript DOM methods. Like so:
useEffect(()=>{
const formElement = document.getElementsByTagName('form')
formElement.item(0).setAttribute('autocomplete', 'off')
},[]);

Putting an element inside Svelte Component

What does putting an element inside Svelte Component mean?
Eg. this code:
const target = document.createElement('div');
// render the component in the new element
const sample = new Sample({ target });
Like, here, in the given linked code, author is doing that:
https://github.com/rspieker/jest-transform-svelte/blob/master/example/test/Sample.spec.js#L8
What does this do? Is it putting Svelte component inside a div? Is it a Svelte syntax to put the element inside the constructor of the Svelte component?
Yes, that snippet is initializing the Svelte component named Sample and rendering it within the target div. The target property of a Svelte component constructor's options parameter is the only required property.
For more information, check out Svelte's components documentation.
It is the place where in your document the component will be rendered. Normally you would use a very specific location like body or a div with a certain id.
In this case however you are not actually rendering a page but merely testing a component so it doesn't matter where the div is.
You can find more info on testing with Jest here https://jestjs.io/

Is the reason props in React shouldn't be changed/mutated is because React wants elements to be as predictable as possible?

From React documentation.
Conceptually, components are like JavaScript functions. They accept
arbitrary inputs (called “props”) and return React elements describing
what should appear on the screen.
Considering:
function Welcome(props) {
return <h1>Hello, {props.name}</h1>;
}
or
class Welcome extends React.Component {
render() {
return <h1>Hello, {this.props.name}</h1>;
}
}
Will give us the ability to do this:
<Welcome name="Luke" />;
<Welcome name="Leia" />;
to use as we wish in the DOM,
Hello, Luke
Hello, Leia
Now when people prescribe props shouldn't be changed, it would make sense the reason is in my thinking would be like the same as changing the values of attributes of an image tag?
HTML:
<img id="Executor" alt="Picture of Executor" src="/somepath/vaders-star-destroyer-executor.jpg"/>
JS:
Meanwhile in a Javascript file a long time ago in a galaxy far, far away...
var imageOfVadersStarDestroyer = document.getElementById('Executor');
imageOfVadersStarDestroyer.src = "/somepath/vaders-star-destroyer-avenger.jpg"
Because if we keeping changing an elements attribute values this can cause confusion and slower renderings?
So is the reason why the prescription is to never change props in React is because is the library is trying to make elements as predictable as possible?
Setting props outside of React is dangerous and should be avoided. Why? The main reason is that it doesn't trigger re-renders. Hence bugs and unexpected behaviour.
Re-rendering
Most of the time, props are data that is store as state in the parent component, which is manipulated by calling setState() (or the second function returned by React.useState()). Once setState() is called, React re-renders and computes what has changed under the hood, with the latest props and state. Manually assigning values to props, therefore won't notify React that the data has changed and something has to be re-rendered.
The good practice
Making props read-only allows React components to be as pure as possible, which is obviously a good practice anyway even when writing plain JS. Data won't be changed unexpectedly and can only be done so by calling setState() (You might have heard of the single source of truth, which is what React is trying to leverage).
Imagine you notice something went wrong in the app and the data shown to the end user is completely different from the source, it would be a pain trying to find out where the data has been manipulated wouldn't it? :)
never change props in React
means that you should never do this.props.name = "userName" because of React's one way data binding, props are read only, to update a component's props, you should pass a function from the parent that will do that ( in the parent ) , or dispatch an action if you're using redux, a change in the props will trigger a re-render
props is a constant in this case. You will always need it in your components.
But there is a cleaner way to write it or even omit it.
Regular way with Function Expression (same as your exemple)
function Welcome(props) {
return <h1>Hello, {props.name}</h1>;
}
ES6 Object Destructing - explicit
function Welcome(props) {
const {name} = pros
return <h1>Hello, {name}</h1>;
}
ES6 Object Destructing - inplicit, cleaner way
function Welcome({name}) {
return <h1>Hello, {name}</h1>;
}
And of course, you can use the class way which requires the usage of this.props.yourAttr
However, in the new version 3 of create-react-app, changed class components to functional components. You can see this exact modification on Github here.
You can need to learn more about destructing assignment in the old and good MDN linked here or an in-depth approach both array and object destructuring here.

setState vs refs in react.js

I created tabs in react and and now on click I have to change the class of the tabs the tabs classes may be as follows:
1:active
2:previousActive
3:alreadySelected
On click of a tab class become active and check whether it is selected before or not using alreadySelected class and active class from the last active tab is remove and if it is not alreadySelected then add alreadySelected.
Code of one tab in react:
var TabBody = React.createClass({
getInitialState: function() {
return {
class: 'tabBody tab activeTab'
}
},
render: function() {
a.tabBody = this;
return (React.createElement('div', {
className: this.state.class,
ref: 'body',
onClick: handleTabClick
},
React.createElement('span', {}, "Body"))
);
}
});
In order to change the class of the tabs I am doing in two ways and want to know which is effective. Code style one:
var bodyClass = (a.tabBody.state.class).split(' ');
var sleeveClass = (a.tabSleeve.state.class).split(' ');
var neckClass = (a.tabNeck.state.class).split(' ');
if (data === 'tabBody') {
bodyClass.push('activeTab');
var str1 = program.arrayToString(bodyClass);
Interfaces.tabBody.setState({
class: str1
});
}
Code Style 2
a.tabBody.refs.body.classList.remove('activeTab');
a.tabBody.refs.body.classList.add('tabPreviewComplete');
a.tabSleeve.refs.body.classList.add('activeTab');
Which style is good for doing this and why?
The point of react is that you do not need to/ should not update DOM directly. The idea behind react is that you render react components (virtual DOM), and that you let react figure out if and how to update DOM.
Changing classes using refs is a very risky strategy: Your component's state is then no longer in sync with actual DOM, which could bring you into debugging nightmares later on. So I would pose that Code Style 2 (even though it works) violates react principles.
One of the few exceptions for using refs, is to add a listener to a DOM component after it is mounted.
The react way is to put the classNames in state.
And do a setState() to update.
And let react do the DOM update,
which is very likely to be way faster, cleaner, and easier to maintain than getting refs, and changing classNames.
ref means you are using the actual DOM and setState means you are saying to react that please update the specific attribute of the component.
every thing is maintain by react.
On the other hand if you use refs it means you are doing every thing your own and react have no concern to your attributes and properties you are updating.

Is my React component being recreated instead of updating?

I am trying to combine Angular and React.js. I have an work example project here I have seen a couple of ways to bring the Angular and React.js together. One of the methods I have seen is to create a directive and create the React component in the link function. For example in the first part of the project to generate the React version(in red) I am using
.directive('reactElementRepeater', function() {
return {
link: function(scope, element) {
var update_react = function(oldVal, newVal){ //Called every time one of the two values change
React.renderComponent(Demo_Element({
numberOfElements: scope.myModel.numberOfElem,
numberInElements: scope.myModel.numberInElem
}), element[0]);
}
scope.$watch('myModel.numberOfElem.length', update_react);
scope.$watch('myModel.numberInElem', update_react);
}
}
});
What I want and what should happen in a React enabled application is for something in the model to be updated, then that update is sent through React and it will alter the DOM as little as possible to reflect that change. It looks like that instead of updating a bit of the DOM this will Create a new React component each time with renderComponent.
React.renderComponent() instantiates the root component, starts the
framework, and injects the markup into a raw DOM element, provided as
the second argument.
Is it actually recreating the elements each time? If that is the case is there a way to alter this so that doesn't happen?
Just to be clear I know about ngReact, I just want to know other ways to speed up Angular with React.
Yes this is fine, it's not mounting the component multiple times.
When you call React.renderComponent() the second argument is the element which react should render the component to. So react notices if you are rendering the same component to a dom element that already contains a mounted instance of the component, and does not re-mount the entire component, it just updates the properties of it instead.
You can see this in action if you make a component with componentDidMount function defined. You'll notice that componentDidMount will only execute the first time renderComponent gets called. And afterwards, subsequent calls to renderComponent on the same target dom element will not call it because the component is already mounted. Likewise getDefaultState and getDefaultProps also only get called on the first renderComponent call.
If you're asking will the render function of the component be called every time the answer is yes. But this is how react works, you want the render function to get called because props might have changed. You can block it from being called by using shouldComponentUpdate (http://facebook.github.io/react/docs/component-specs.html#updating-shouldcomponentupdate) and returning false. However react developers recommend you don't use this to block render calls unless you have specific performance problems - most of the time it should be fine to just let the render call execute as it wont cause any slow dom updates unless things have actually changed.

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