One option would be to use the prompt() function, which displays a modal dialog through which user input can be entered and acquired. The prompt() method also allows you to supply a custom greeting, which can be passed as the first argument like so:

    const enteredName = prompt('Please enter your name')

Integrating this with your existing react component can be done in a number of ways - one approach might be as follows:

/* Definition of handleClick in component */
handleClick = (event) => {

    /* call prompt() with custom message to get user input from alert-like dialog */
    const enteredName = prompt('Please enter your name')

    /* update state of this component with data provided by user. store data
       in 'enteredName' state field. calling setState triggers a render of
       this component meaning the enteredName value will be visible via the
       updated render() function below */
    this.setState({ enteredName : enteredName })
}

render: function() {
    return (
      <div>

        {/* For demonstration purposes, this is how you can render data 
            previously entered by the user */ }
        <p>Previously entered user name: { this.state.enteredName }</p>

        <input type="text" onChange={ this.handleChange } />
        <input
          type="button"
          value="Alert the text input"
          onClick={this.handleClick}
        />
      </div>
    );
  }
});

For class component

In app.js

import React from "react";
import Modal from "./Component/Modal";
import "./styles.css";
class App extends React.Component {
  state = {
    show: false
  };
  showModal = e => {
    this.setState({
      show: !this.state.show
    });
  };
  render() {
    return (
      <div className="App">
        <button
          class="toggle-button"
          id="centered-toggle-button"
          onClick={e => {
            this.showModal(e);
          }}
        >
          {" "}
          show Modal{" "}
        </button>

        <Modal onClose={this.showModal} show={this.state.show}>
          Lorem ipsum dolor sit amet, consectetur adipisicing elit. Nobis
          deserunt corrupti, ut fugit magni qui quasi nisi amet repellendus non
         
        </Modal>
      </div>
    );
  }
}

export default App;

In component/modal

Make file named index.js

import React from "react";
import "./modal.css";
import PropTypes from "prop-types";

export default class Modal extends React.Component {
  onClose = e => {
    this.props.onClose && this.props.onClose(e);
  };
  render() {
    if (!this.props.show) {
      return null;
    }
    return (
      <div class="modal" id="modal">
        <h2>Modal Window</h2>
        <div class="content">{this.props.children}</div>
        <div class="actions">
          <button class="toggle-button" onClick={this.onClose}>
            close
          </button>
        </div>
      </div>
    );
  }
}
Modal.propTypes = {
  onClose: PropTypes.func.isRequired,
  show: PropTypes.bool.isRequired
};

Make file modal.css

html,
body {
  height: 100%;
}
body {
  background: #eee;
  display: flex;
  justify-content: center;
  align-items: center;
}
.modal {
  width: 500px;
  background: white;
  border: 1px solid #ccc;
  transition: 1.1s ease-out;
  box-shadow: -2rem 2rem 2rem rgba(0, 0, 0, 0.2);
  filter: blur(0);
  transform: scale(1);
  opacity: 1;
  visibility: visible;
}
.modal.off {
  opacity: 0;
  visibility: hidden;
  filter: blur(8px);
  transform: scale(0.33);
  box-shadow: 1rem 0 0 rgba(0, 0, 0, 0.2);
}
@supports (offset-rotation: 0deg) {
  offset-rotation: 0deg;
  offset-path: path("M 250,100 S -300,500 -700,-200");
  .modal.off {
    offset-distance: 100%;
  }
}
@media (prefers-reduced-motion) {
  .modal {
    offset-path: none;
  }
}
.modal h2 {
  border-bottom: 1px solid #ccc;
  padding: 1rem;
  margin: 0;
}
.modal .content {
  padding: 1rem;
}
.modal .actions {
  border-top: 1px solid #ccc;
  background: #eee;
  padding: 0.5rem 1rem;
}
.modal .actions button {
  border: 0;
  background: #78f89f;
  border-radius: 5px;
  padding: 0.5rem 1rem;
  font-size: 0.8rem;
  line-height: 1;
}
#centered-toggle-button {
  position: absolute;
}

This is an example of modal with class component. Please check if this helps you.

@Python

Second example

You can try this, if that does not work. This is bit easy also.

Using react bootstrap module. In App.js

import React from 'react';  
import './App.css';  
import { Button,Modal} from 'react-bootstrap';  
class App extends React.Component {  
  constructor(){  
    super();  
    this.state={  
      show:false  
    }  
  }  
  handleModal(){  
    this.setState({show:!this.state.show})  
  }  
  render(){  
    return (  
      <div>  
        <h2 align='center'>Example of Modal in Reactjs</h2>  
        <div className="modalClass">  
          <Button onClick={()=>this.handleModal()}>Click To Open Modal</Button>  
        </div>  
          
        <Modal show={this.state.show} onHide={()=>this.handleModal()}>  
          <Modal.Header closeButton>This is a Modal Heading</Modal.Header>  
          <Modal.Body>This is a Modal Body</Modal.Body>  
          <Modal.Footer>  
            <Button onClick={()=>this.handleModal()}>Close</Button>  
            <Button onClick={()=>this.handleModal()}>Save</Button>  
          </Modal.Footer>  
        </Modal>  
      </div>  
    )  
  }  
}  
export default App; 

In css file just add

.modalClass {  
  text-align: center;  
  margin-top: 100px;  
} 

You can do it easily with react-modal. Here is an example, as a bonus I added closing modal when clicking on opened image. Image src and its description should be passed to popUp component as props in data object.

PopUp.js:

import React from 'react';
import Modal from 'react-modal';
import ModalButton from './modal-button';
import style from './style.css';

class PopUp extends React.Component {
  constructor() {
    super();

    this.state = { modalOpened: false };
    this.toggleModal = this.toggleModal.bind(this);
  }

  toggleModal() {
    this.setState(prevState => ({ modalOpened: !prevState.modalOpened }));
  }

  render() {
    const { data } = this.props;
    return (
      <div className={style.modalWrapper}>
        <ModalButton handleClick={this.toggleModal}>
          click to open modal
        </ModalButton>
        <Modal
          className={{ base: [style.base]}}
          overlayClassName={{ base: [style.overlayBase] }}
          isOpen={this.state.modalOpened}
          onRequestClose={this.toggleModal}
          contentLabel="Modal with image"
        >
          <img
            onClick={this.toggleModal}
            src={data.src}
            alt='image displayed in modal'
          />
          <span className={style.text}>{data.description}</span>
        </Modal>
      </div>
    );
  }
}

export default PopUp;

And separated button, as you requested:

import React from 'react';
import style from './style.css';

const ModalButton = props => (
  <button className={style.button} onClick={props.handleClick}>
    {props.children}
  </button>);

export default ModalButton;