Develop Reference

getBoundingClientRect() on two React components and check if they overlap onScroll

I want to get a ref, more specifically a getBoundingClientRect() on the <Header/> and <Testimonials/> component. I then want to watch for a scroll event and check if the two components ever overlap. Currently, my overlap variable never flips to true even if what appears on the page is that the two components are overlaping.
const [isIntersecting, setIsIntersecting] = useState(false)
const header = useRef(null)
const testimonials = useRef(null)
const scrollHandler = _ => {
let headerRect = header.current.getBoundingClientRect();
let testiRect = testimonials.current.getBoundingClientRect();
let overlap = !(headerRect.right < testiRect.left ||
headerRect.left > testiRect.right ||
headerRect.bottom < testiRect.top ||
headerRect.top > testiRect.bottom)
console.log(overlap) // never flips to true
};
useEffect(() => {
window.addEventListener("scroll", scrollHandler, true);
return () => {
window.removeEventListener("scroll", scrollHandler, true);
};
}, []);
const App = () => {
return (
<div className="App">
<Header />
<LandingPage />
<div style={{ height: '100vh', backgroundColor: 'black', color: 'white' }}>
</div>
<AboutPage />
<TestimonialsPage />
<Footer />
</div>
);
}

First: Components can't receive directly a ref prop, unless you are wrapping the Component itself in a React.forwardRef wrapper:
const Component = React.forwardRef((props, ref) => (
<button ref={ref}>
{props.children}
</button>
));
// Inside your Parent Component:
const ref = useRef();
<Component ref={ref}>Click me!</Component>;
Second: you can also pass a ref down to a child as a standard prop, but you can't call that prop ref since that's a special reserved word just like the key prop:
const Component= (props) => (
<button ref={props.myRef}>
{props.children}
</button>
);
// Inside your Parent Component
const ref = useRef();
<Component myRef={ref}>Click me!</Component>;
This works perfectly fine, and if it's a your personal project you
might work like this with no issues, the only downside is that you
have to use custom prop name for those refs, so the code gets harder to
read and to mantain, especially if it's a shared repo.
Third: Now that you learnt how to gain access to the DOM node of a child Component from its parent, you must know that even if usually it's safe to perform manipulations on those nodes inside a useEffect ( or a componentDidMount ) since they are executed once the DOM has rendered, to be 100% sure you will have access to the right DOM node it's always better using a callback as a ref like this:
const handleRef = (node) => {
if (node) //do something with node
};
<Component ref={handleRef}/>
Basically your function hanldeRef will be called by React during
DOM node render by passing the node itself as its first parameter,
this way you can perform a safe check on the node, and be sure it's
100% valorized when you are going to perform your DOM manipulation.
Concerning your specific question about how to access the getBoundingClientRect of a child Component DOM node, I made a working example with both the approaches:
https://stackblitz.com/edit/react-pqujuz

You'll need to define each of your components as Forwarding Refs, eg
const Header = forwardRef<HTMLElement>((_, ref) => (
<header ref={ref}>
<h1>I am the header</h1>
</header>
));
You can then pass a HTMLElement ref to your components to refer to later
const headerRef = useRef<HTMLElement>(null);
const scrollHandler = () => {
console.log("header position", headerRef.current?.getBoundingClientRect());
};
useEffect(() => {
window.addEventListener("scroll", scrollHandler);
return () => {
window.removeEventListener("scroll", scrollHandler);
};
}, []);
return (
<Header ref={headerRef} />
);
I'm using TypeScript examples since it's easier to translate back down to JS than it is to go up to TS

Are props passed from parent to child by default with `this`?

I am learning through an open source project here. I have deployed it and it works. So the below pasted code is valid for sure.
I was looking at a Header component in Header.js:
class Header extends React.Component {
state = {
open: false,
};
render() {
const {
classes,
toggleDrawerOpen,
margin,
turnDarker,
} = this.props;
return (
.... some code ....
)
I see that classes is passed as a prop from the parent. So I looked into the parent component, Dashboard. Here is the code:
import { Header, Sidebar, BreadCrumb } from './../../components';
import { toggleAction, openAction, playTransitionAction } from './../../actions/UiActions';
import styles from './appStyles-jss';
class Dashboard extends React.Component {
state = {
transform: 0,
};
componentDidMount = () => {
// Scroll content to top
const mainContent = document.getElementById('mainContent');
mainContent.addEventListener('scroll', this.handleScroll);
// Set expanded sidebar menu
const currentPath = this.props.history.location.pathname;
this.props.initialOpen(currentPath);
// Play page transition
this.props.loadTransition(true);
// Execute all arguments when page changes
this.unlisten = this.props.history.listen(() => {
mainContent.scrollTo(0, 0);
setTimeout(() => {
this.props.loadTransition(true);
}, 500);
});
}
componentWillUnmount() {
const mainContent = document.getElementById('mainContent');
mainContent.removeEventListener('scroll', this.handleScroll);
}
handleScroll = (event) => {
const scoll = event.target.scrollTop;
this.setState({
transform: scoll
});
}
render() {
const {
classes, // classes is here
route,
toggleDrawer,
sidebarOpen,
loadTransition,
pageLoaded
} = this.props;
const darker = true;
return (
<div className={classes.appFrameInner}>
// NOTE: Header component is here but I don't see how classes is passed to it.
<Header toggleDrawerOpen={toggleDrawer} turnDarker={this.state.transform > 30 && darker} margin={sidebarOpen} />
<Sidebar
open={sidebarOpen}
toggleDrawerOpen={toggleDrawer}
loadTransition={loadTransition}
turnDarker={this.state.transform > 30 && darker}
/>
<main className={classNames(classes.content, !sidebarOpen && classes.contentPadding)} id="mainContent">
<div className={classes.bgbar} />
</main>
</div>
);
}
}
You can see that the classes prop is passed from Dashboard's parent. However, I was expecting some syntax that shows it is passed into the child Header component.
See the "NOTE" line in the code, nothing was said about passing the entire props to Header component or passing the const classes specifically to Header.
How is classes passed from parent (Dashbaord) to child (Header)?

The classes prop is not passed from parent Dashboard to child Header.
The classes prop is made available directly to your Header component using the wrapping withStyles HOC when exporting your component:
export default withStyles(styles)(Header);
This approach is commonly known as CSS-in-JS and you can read more details in the material-ui styles documentation.

How to find focused React component? (like document.activeElement)

If you had 500 components, each with a ref, how would you find which component has the user's focus? All components with a ref are focusable elements like <input />, <textarea />, etc. For simplicity, all of the refs are accessible from a single top-level component.
This is very simple if your React components have classNames - but if you want to find the ref of a document.activeElement, if there some way to achieve that without having to resort to classNames?
To touch on why we don't have classNames, we're using JSS via emotion. To have to manually assign everything a className just for this purpose would be rather absurd. No obvious alternative has occurred to me.

This may be a good use case for a custom hook that hooks into the native DOM methods to track focus events and then returns the active element. This will log the active element every time a new element receives focus:
const useActiveElement = () => {
const [active, setActive] = useState(document.activeElement);
const handleFocusIn = (e) => {
setActive(document.activeElement);
}
useEffect(() => {
document.addEventListener('focusin', handleFocusIn)
return () => {
document.removeEventListener('focusin', handleFocusIn)
};
}, [])
return active;
}
const App = () => {
const focusedElement = useActiveElement();
useEffect(() => {
if (focusedElement) {
focusedElement.value && console.log(focusedElement.value);
}
console.log(focusedElement);
}, [focusedElement])
return (
<div>
<input type="text"/>
<button>Button</button>
</div>
)
}
However, correlating this element with your refs could prove tricky, as you'd need to keep a collection of the refs to search through which would probably involve giving each element its own callback ref to store in an Array or something similar. But depending on what you need to do with the element once it's focused, it may not be necessary. For example, the code I posted above will log the value of the input if it exists. It would help to know more specifically what your use case is for tracking this data.

Do you want something like that, react gives you an option to track/focus using ref
import React, { Component } from "react";
class App extends Component {
constructor(props) {
super(props);
// create a ref to store the textInput DOM element
this.textInput = React.createRef();
this.button = React.createRef();
this.textarea = React.createRef();
this.focusTextInput = this.focusTextInput.bind(this);
}
focusTextInput() {
// this.textInput.current.focus();
this.button.current.focus();
// this.textarea.current.focus();
}
render() {
return (
<div>
<input type="text" ref={this.textInput} />
<button ref={this.button}>something</button>
<textarea ref={this.textarea}></textarea>
<input
type="button"
value="Focus the text input"
onClick={this.focusTextInput}
/>
</div>
);
}
}
export default App;
You can get a detail here

In react, how to get noticed when children change?

I am making this class called Scrollable which enables scrolling if the width/height of the children elements exceeds a certain value. Here is the code.
import React, { Component } from 'react';
const INITIAL = 'initial';
class Scrollable extends Component {
render() {
let outter_styles = {
overflowX: (this.props.x? 'auto': INITIAL),
overflowY: (this.props.y? 'auto': INITIAL),
maxWidth: this.props.width || INITIAL,
maxHeight: this.props.height || INITIAL,
};
return (
<div ref={el => this.outterEl = el} style={outter_styles}>
<div ref={el => this.innerEl = el}>
{this.props.children}
</div>
</div>
);
}
};
export default Scrollable;
// To use: <Scrollable y><OtherComponent /></Scrollable>
This works great. Except now I wish to add one more functionality which makes the scrollable always scroll to the bottom. I have some idea of how to do it:
this.outterEl.scrollTop = this.innerEl.offsetHeight;
But this only need to be called when this.props.children height changes. Is there any idea on how to achieve this?
Thanks in advance.

I would recommend a package element-resize-detector. It is not React-specific but you can easily build a high-order component around it or integrate your Scrollable component with it.

Now I have an idea of solving this.
Since I am using react-redux. The problem is that I could not use lifecycle hooks on this Scrollable component since this.props.children might not necessarily be changed when the content is updated.
One way to achieve this is to make Scroll component aware of the corresponding values in the redux state. So that when that relevant value is updated, we can scroll down to the bottom.
Scrollable component:
import React, { Component } from 'react';
const INITIAL = 'initial';
class Scrollable extends Component {
componentWillUpdate(){
if(this.props.autoScroll){
// only auto scroll when the scroll is already at bottom.
this.autoScroll = this.outterEl.scrollHeight - this.outterEl.scrollTop - Number.parseInt(this.props.height) < 1;
}
}
componentDidUpdate(){
if(this.autoScroll) this.outterEl.scrollTop = this.outterEl.scrollHeight;
}
render() {
let styles = {
overflowX: (this.props.x? 'auto': INITIAL),
overflowY: (this.props.y? 'auto': INITIAL),
maxWidth: this.props.width || INITIAL,
maxHeight: this.props.height || INITIAL,
};
return (
<div ref={el => this.outterEl = el} style={styles}>
<div ref={el => this.innerEl = el}>
{this.props.children}
</div>
</div>
);
}
};
export default Scrollable;
Scrollable container:
import { connect } from 'react-redux';
import Scrollable from '../components/Scrollable';
const mapStateToProps = (state, ownProps) => Object.assign({
state: state[ownProps.autoScroll] || false
}, ownProps);
export default connect(mapStateToProps)(Scrollable)
With this, Scrollable's life cycle hooks will be called when the corresponding state changes.

How to access a DOM element in React? What is the equilvalent of document.getElementById() in React

How do I select certain bars in react.js?
This is my code:
var Progressbar = React.createClass({
getInitialState: function () {
return { completed: this.props.completed };
},
addPrecent: function (value) {
this.props.completed += value;
this.setState({ completed: this.props.completed });
},
render: function () {
var completed = this.props.completed;
if (completed < 0) { completed = 0 };
return (...);
}
I want to use this React component:
var App = React.createClass({
getInitialState: function () {
return { baction: 'Progress1' };
},
handleChange: function (e) {
var value = e.target.value;
console.log(value);
this.setState({ baction: value });
},
handleClick10: function (e) {
console.log('You clicked: ', this.state.baction);
document.getElementById(this.state.baction).addPrecent(10);
},
render: function () {
return (
<div class="center">Progress Bars Demo
<Progressbar completed={25} id="Progress1" />
<h2 class="center"></h2>
<Progressbar completed={50} id="Progress2" />
<h2 class="center"></h2>
<Progressbar completed={75} id="Progress3" />
<h2 class="center"></h2>
<span>
<select name='selectbar' id='selectbar' value={this.state.baction} onChange={this.handleChange}>
<option value="Progress1">#Progress1</option>
<option value="Progress2">#Progress2</option>
<option value="Progress3">#Progress3</option>
</select>
<input type="button" onClick={this.handleClick10} value="+10" />
<button>+25</button>
<button>-10</button>
<button>-25</button>
</span>
</div>
)
}
});
I want to execute the handleClick10 function and perform the operation for my selected progressbar.
But the result I get is:
You clicked: Progress1
TypeError: document.getElementById(...) is null
How do I select the certain Element in react.js?

You can do that by specifying the ref
EDIT: In react v16.8.0 with function component, you can define a ref with useRef. Note that when you specify a ref on a function component, you need to use React.forwardRef on it to forward the ref to the DOM element of use useImperativeHandle to to expose certain functions from within the function component
Ex:
const Child1 = React.forwardRef((props, ref) => {
return <div ref={ref}>Child1</div>
});
const Child2 = React.forwardRef((props, ref) => {
const handleClick= () =>{};
useImperativeHandle(ref,() => ({
handleClick
}))
return <div>Child2</div>
});
const App = () => {
const child1 = useRef(null);
const child2 = useRef(null);
return (
<>
<Child1 ref={child1} />
<Child1 ref={child1} />
</>
)
}
EDIT:
In React 16.3+, use React.createRef() to create your ref:
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.myRef = React.createRef();
}
render() {
return <div ref={this.myRef} />;
}
}
In order to access the element, use:
const node = this.myRef.current;
DOC for using React.createRef()
EDIT
However facebook advises against it because string refs have some issues, are considered legacy, and are likely to be removed in one of the future releases.
From the docs:
Legacy API: String Refs
If you worked with React before, you might be
familiar with an older API where the ref attribute is a string, like
"textInput", and the DOM node is accessed as this.refs.textInput. We
advise against it because string refs have some issues, are considered
legacy, and are likely to be removed in one of the future releases. If
you're currently using this.refs.textInput to access refs, we
recommend the callback pattern instead.
A recommended way for React 16.2 and earlier is to use the callback pattern:
<Progressbar completed={25} id="Progress1" ref={(input) => {this.Progress[0] = input }}/>
<h2 class="center"></h2>
<Progressbar completed={50} id="Progress2" ref={(input) => {this.Progress[1] = input }}/>
<h2 class="center"></h2>
<Progressbar completed={75} id="Progress3" ref={(input) => {this.Progress[2] = input }}/>
DOC for using callback
Even older versions of react defined refs using string like below
<Progressbar completed={25} id="Progress1" ref="Progress1"/>
<h2 class="center"></h2>
<Progressbar completed={50} id="Progress2" ref="Progress2"/>
<h2 class="center"></h2>
<Progressbar completed={75} id="Progress3" ref="Progress3"/>
In order to get the element just do
var object = this.refs.Progress1;
Remember to use this inside an arrow function block like:
print = () => {
var object = this.refs.Progress1;
}
and so on...

For getting the element in react you need to use ref and inside the function you can use the ReactDOM.findDOMNode method.
But what I like to do more is to call the ref right inside the event
<input type="text" ref={ref => this.myTextInput = ref} />
This is some good link to help you figure out.

With newer versions of React you can use and manipulate the DOM via hooks like this:
import React, { useEffect, useRef } from "react";
const MyComponent = () => {
const myContainer = useRef(null);
useEffect(() => {
console.log("myContainer..", myContainer.current);
});
return (
<>
<h1>Ref with react</h1>
<div ref={myContainer}>I can use the DOM with react ref</div>
</>
);
};
export default MyComponent;
Whenever you want to access your DOM element just use myContainer.current

You can replace
document.getElementById(this.state.baction).addPrecent(10);
with
this.refs[this.state.baction].addPrecent(10);
<Progressbar completed={25} ref="Progress1" id="Progress1"/>

Disclaimer: While the top answer is probably a better solution, as a beginner it's a lot to take in when all you want is something very simple. This is intended as a more direct answer to your original question "How can I select certain elements in React"
I think the confusion in your question is because you have React components which you are being passed the id "Progress1", "Progress2" etc. I believe this is not setting the html attribute 'id', but the React component property. e.g.
class ProgressBar extends React.Component {
constructor(props) {
super(props)
this.state = {
id: this.props.id <--- ID set from <ProgressBar id="Progress1"/>
}
}
}
As mentioned in some of the answers above you absolutely can use document.querySelector inside of your React app, but you have to be clear that it is selecting the html output of your components' render methods. So assuming your render output looks like this:
render () {
const id = this.state.id
return (<div id={"progress-bar-" + id}></div>)
}
Then you can elsewhere do a normal javascript querySelector call like this:
let element = document.querySelector('#progress-bar-Progress1')

You have to follow two different ways to do it in Class and Functional components.
For class components
<input type="text" ref={ref => this.myTextInput = ref} />
Look at the above code. Use "ref" attribute to refer to the relevant element. Then you will be able to refer to that element using that reference. In this example, I can use "this.myTextInput" to refer to the above input element.
For functional components
const textInput = useRef(null)
Use the "useRef" hook and set that variable name as the value of the "ref" attribute of the element you want to refer to (like below).
<input type="text" ref={textInput} />
An example for this on functional components.
import React, {useRef} from 'react'
function CustomTextInput(props) {
// textInput must be declared here so the ref can refer to it
const textInput = useRef(null);
function handleClick() {
textInput.current.focus();
}
return (
<div>
<input type="text" ref={textInput} />
</div>
);
}
Want to learn more? Here you go

Since React uses JSX code to create an HTML we cannot refer dom using regulation methods like documment.querySelector or getElementById.
Instead we can use React ref system to access and manipulate Dom as shown in below example:
constructor(props){
super(props);
this.imageRef = React.createRef(); // create react ref
}
componentDidMount(){
**console.log(this.imageRef)** // acessing the attributes of img tag when dom loads
}
render = (props) => {
const {urls,description} = this.props.image;
return (
<img
**ref = {this.imageRef} // assign the ref of img tag here**
src = {urls.regular}
alt = {description}
/>
);
}
}

In my case, I wasn't able to use ref because elements were somewhere between many child components and I have to access them by class and id instead of ref. So, trying with useEffect hook didn't work as it can't find the element:
useEffect(() => {
const el1 = document.querySelector('.el1')
const el2 = document.querySelector('.el2')
}, [])
The element is undefined because when it is mounted the children components also doesn't mounted before this parent component.
So, what I did is to use timeout:
useEffect(() => {
const timer = setTimeout(() => {
const el1 = document.querySelector('.el1')
const el2 = document.querySelector('.el2')
},500)
return () => {
clearTimeout(timer)
}
}, [])
Now, it worked fine. It found the DOM and I was able to manipulate with them. Hope, this helps someone!

The equivalent of document.getElementById() in React is document.querySelector().