Popper.js is a dependency of Bootstrap 4 which is used for showing popups. It is a peer dependency of bootstrap meaning it is something that bootstrap requires but doesn't include with itself when installed. So to install popper.js run

npm install popper.js --save

It is setup as a peer dependency because some people just use the css of Bootstrap without the javascript. jQuery and popper.js are the peer dependencies which needs to be installed separately. If you require the javascript of Bootstrap you need to install jQuery and popper.js alongside Bootstrap 4.

Bootstrap 5 requires "Popper.js Core", not Popper.js. You should run this instead:

npm install @popperjs/core --save

(thanks @Kyouma for adding this in the comments)

For bootstrap 5 you are to

npm users: npm install @popperjs/core --save

for yarn users: yarn add @popperjs/core

then import bootstrap like so:

import 'bootstrap/dist/js/bootstrap.bundle';

It's simple you can Visit this, And save the file as popper.min.js

then import it.

import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';
import 'bootstrap/dist/css/bootstrap.css';
import './index.css';
import 'bootstrap/dist/js/popper.min.js';
global.jQuery = require('jquery');
require('bootstrap');