There are multiple use cases in the modern UX when we are required to use infinite scrolling. Previously, devs are using the height of the viewport and element to find out the intersection of the element is in the viewport. The main issue in the same is that the function that calculations will be executed on the main queue so it makes your app a bit slow and it was bit unreliable. A few days ago I came across the Intersection Observer API. Which can be used in the following applications:

• Lazy-loading of images or other content as a page is scrolled.
• Implementing "infinite scrolling" websites, where more and more content is loaded and rendered as you scroll so that the user doesn't have to flip through pages.
• Reporting of visibility of advertisements to calculate ad revenues.
• Deciding whether or not to perform tasks or animation processes based on whether or not the user will see the result.

The Intersection Observer API provides a way to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document's viewport.

Source code is available at https://github.com/dhairyanadapara/infinite-scoller-example

Demo link: https://dhairyanadapara.github.io/infinite-scoller-example/

Let's start with the solution.

import React, { Component } from "react";
import "./App.css";

class App extends Component {
    constructor(props) {
        super(props);
        this.state = {
            arr: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15],
        };
    }
    componentDidMount() {
        this.createObserver();
    }

    createObserver = () => {
        let options = {
            root: null,
            rootMargin: " 40px",
            threshold: [0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0],
        };
        const boxElement = document.getElementById("loading");
        const observer = new IntersectionObserver(
            this.handleIntersect,
            options
        );
        observer.observe(boxElement);
    };

    handleIntersect = (entries, observer) => {
        const { arr } = this.state;
        entries.forEach((entry) => {
            console.log(entry.intersectionRatio);
            if (entry.intersectionRatio > 0) {
                this.setState({
                    arr: arr.concat([
                        10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21,
                    ]),
                });
            }
        });
    };

    render() {
        const { arr } = this.state;
        return (
            <div className="App" id="app">
                <div id="infinite-container">
                    <div class="cards-list" id="card-list">
                        {arr.map((x) => (
                            <div class="card 1">
                                <div class="card_image">
                                    {" "}
                                    <img src="https://i.redd.it/b3esnz5ra34y.jpg" />
                                </div>
                                <div class="card_title title-white">
                                    <p>Card Title</p>
                                </div>
                            </div>
                        ))}
                    </div>
                    <div id="loading" style={{ height: "100px" }}>
                        Loading
                    </div>
                </div>
            </div>
        );
    }
}

export default App;

As you can see we have used the react class component so it will be easy to understand. You can use functional components also.

Let's start with understanding the observer initialization.

createObserver = () => {
    let options = {
        root: null,
        rootMargin: " 40px",
        threshold: [0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0],
    };
    const boxElement = document.getElementById("loading");
    const observer = new IntersectionObserver(this.handleIntersect, options);
    observer.observe(boxElement);
};

IntersectionObserver takes 2 arguments.

1. options
   options are the configuration for Intersection Observer. It has 3 properties:

   1. root: The element which you want to use as the viewport. If you want to use the browser's viewport pass null.
   2. rootMargin : Offset is added to the target rectangle while calculating intersections
   3. threshold: A list of thresholds, sorted in increasing numeric order. The callback will be called when intersectionRatio passes the threshold

2. callback
   callback has 2 argument:

   1. entries list of IntersectionObserverEntry, which describes the intersection between target and root element
   2. observer IntersectionObserver object same we have created in createObserver

Here we are observing the loading element which will be at bottom of the card list. In our case, we have only 1 target element in the observer so we will get only 1 object in entries. If you have multiple target elements targets to the same observers you will get more entries.

handleIntersect = (entries, observer) => {
    const { arr } = this.state;
    entries.forEach((entry) => {
        if (entry.intersectionRatio > 0) {
            this.setState({
                arr: arr.concat([
                    10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21,
                ]),
            });
        }
    });
};

IntersectionObserverEntry object have multiple attributes like boundingClientRect,intersectionRatio,intersectionRect,isIntersecting,rootBounds,target,
time.

The main attributes are:

• intersectionRatio: returns the percentage of intersectionRect to the boundingClientRect
• isIntersecting: return if target and root are intersecting or not.
• target: this is an important attribute when we have multiple targets attached to the same observer

In the above function we have iterated over the entries and checked if the intersection ratio is more than 0 is not means the target element has an intersection with root or viewport is happened or not. As you can see we are observing the element with id loading which is placed at bottom of the card-list element. So what will happen when we scroll down and reach the loading element it intersection will happen and the state will be updated accordingly.

In this case, we are not doing any API calls so data is getting updated quickly. In case of fetch request, it would be better to use the rootMargin. You can also update the threshold based on requirements.