you can put your script in an html file before react is being called.

solution depends on scenario. like in my case, i had to load a calendly embed inside a react component.

calendly looks for a div and reads from it's data-url attribute and loads an iframe inside the said div.

it is all good when you first load the page: first, div with data-url is rendered. then calendly script is added to body. browser downloads and evaluates it and we all go home happy.

problem comes when you navigate away and then come back into the page. this time the script is still in body and browser doesn't re-download & re-evaluate it.

fix:

1. on componentwillunmount find and remove the script element. then on re mount, repeat the above steps.
2. enter $.getscript. it is a nifty jquery helper that takes a script uri and a success callback. once the script it loaded, it evaluates it and fires your success callback. all i have to do is in my componentdidmount $.getscript(url). my render method already has the calendly div. and it works smooth.

i saw the same problem, until i found this package, quite easy to implement, i hope it works as it worked for me :)

https://github.com/gumgum/react-script-tag

import react from 'react';
import script from '@gumgum/react-script-tag';

import './app.css';

function app() {
  return (
    <div >

      <h1> graphs</h1>
      <div class="flourish-embed flourish-network" data-src="visualisation/8262420">
        <script  src"your script"
        </script>
      </div>
    </div>
  );
}

export default app;

for multiple scripts, use this

var loadscript = function(src) {
  var tag = document.createelement('script');
  tag.async = false;
  tag.src = src;
  document.getelementsbytagname('body').appendchild(tag);
}
loadscript('//cdnjs.com/some/library.js')
loadscript('//cdnjs.com/some/other/library.js')

for a more complete usescript implementation that supports loading status and error handling, check out this from usehooks.

usage

function app() {
  const status = usescript(
    "https://pm28k14qlj.codesandbox.io/test-external-script.js"
  );
  return (
    <div>
      <div>
        script status: <b>{status}</b>
      </div>
      {status === "ready" && (
        <div>
          script function call response: <b>{test_script.start()}</b>
        </div>
      )}
    </div>
  );
}

hook

function usescript(src) {
  // keep track of script status ("idle", "loading", "ready", "error")
  const [status, setstatus] = usestate(src ? "loading" : "idle");
  useeffect(
    () => {
      // allow falsy src value if waiting on other data needed for
      // constructing the script url passed to this hook.
      if (!src) {
        setstatus("idle");
        return;
      }
      // fetch existing script element by src
      // it may have been added by another intance of this hook
      let script = document.queryselector(`script[src="${src}"]`);
      if (!script) {
        // create script
        script = document.createelement("script");
        script.src = src;
        script.async = true;
        script.setattribute("data-status", "loading");
        // add script to document body
        document.body.appendchild(script);
        // store status in attribute on script
        // this can be read by other instances of this hook
        const setattributefromevent = (event) => {
          script.setattribute(
            "data-status",
            event.type === "load" ? "ready" : "error"
          );
        };
        script.addeventlistener("load", setattributefromevent);
        script.addeventlistener("error", setattributefromevent);
      } else {
        // grab existing script status from attribute and set to state.
        setstatus(script.getattribute("data-status"));
      }
      // script event handler to update status in state
      // note: even if the script already exists we still need to add
      // event handlers to update the state for *this* hook instance.
      const setstatefromevent = (event) => {
        setstatus(event.type === "load" ? "ready" : "error");
      };
      // add event listeners
      script.addeventlistener("load", setstatefromevent);
      script.addeventlistener("error", setstatefromevent);
      // remove event listeners on cleanup
      return () => {
        if (script) {
          script.removeeventlistener("load", setstatefromevent);
          script.removeeventlistener("error", setstatefromevent);
        }
      };
    },
    [src] // only re-run effect if script src changes
  );
  return status;
}

i had raw html string with javascript/jquery i installed npm library dangerously-set-html-content npm i dangerously-set-html-content

import innerhtml from 'dangerously-set-html-content'
<div>
<innerhtml html={html}/>
</div>

or

import innerhtml from 'dangerously-set-html-content'
const renderhtml=`<!doctype html><html lang="en"><head><meta charset="utf-8"><title> is not defined</title>$(document).ready(function(){    $("button").click(function(){        alert("jquery is working perfectly.");    });      });</script></head><body>    <button type="button">test jquery code</button></body></html>`

<div>
<innerhtml html={renderhtml}/>
</div>

make sure you add jquery cdn to public/index.html file

 <script          src="https://code.jquery.com/jquery-3.3.1.min.js"          integrity="sha256-fgpcb/kjqllnfou91ta32o/nmzxltwro8qtmkmrdau8="          crossorigin="anonymous"          async="true"        ></script>

componentdidmount() {
  const head = document.queryselector("head");
  const script = document.createelement("script");
  script.setattribute(
    "src",
    "https://assets.calendly.com/assets/external/widget.js"
  );
  head.appendchild(script);
}

just add in body in html file

<script src="https://unpkg.com/react-dom@17/umd/react-dom.development.js" crossorigin></script>

honestly, for react - don't bother with messing around adding <script> tags to your header. it's a pain in the ass to get a callback when they have loaded fully. instead, use a package like @charlietango/usescript to load the script when you need it and get a status update when it is completed.

example usage:

import react from 'react'
import usescript, { scriptstatus } from '@charlietango/use-script'
 
const component = () => {
  const [ready, status] = usescript('https://api.google.com/api.js')
 
  if (status === scriptstatus.error) {
    return <div>failed to load google api</div>
  }
 
  return <div>google api ready: {ready}</div>
}
 
export default component

ps. if you're using redux to tell other components when your script has loaded, and are using redux-persist like i was, don't forget to include a modifier on your redux-persist setup that always sets the script loaded redux value to false in the redux backup.

i recently faced the issue, tried multiple solutions given here, at last sattled with iframe, iframe seems to work seamlessly if it you are trying to integrate a js plugin on a specific screen

    <iframe
      id="xxx"
      title="xxx"
      width="xxx"
      height="xxx"
      frameborder="value"
      allowtransparency
      srcdoc={`
          <!doctype html>
          <html>
          <head>
              <title>chat bot</title>
              <meta charset="utf-8">
              <meta name="viewport" content="width=device-width">
          </head>
          <body style="width:100%">
              <script type="text/javascript"> 
                ......
                            </script>  
          </body>
          </html>
          `}
 />

a bit late to the party but i decided to create my own one after looking at @alex macmillan answers and that was by passing two extra parameters; the position in which to place the scripts such as or and setting up the async to true/false, here it is:

import { useeffect } from 'react';

const usescript = (url, position, async) => {
  useeffect(() => {
    const placement = document.queryselector(position);
    const script = document.createelement('script');

    script.src = url;
    script.async = typeof async === 'undefined' ? true : async;

    placement.appendchild(script);

    return () => {
      placement.removechild(script);
    };
  }, [url]);
};

export default usescript;

the way to call it is exactly the same as shown in the accepted answer of this post but with two extra(again) parameters:

// first string is your url
// second string can be head or body
// third parameter is true or false.
usescript("string", "string", bool);

very similar to other answers just using default values to clean up undefined checks

import { useeffect } from 'react'

const usescript = (url, selector = 'body', async = true) => {
  useeffect(() => {
    const element = document.queryselector(selector)
    const script = document.createelement('script')
    script.src = url
    script.async = async
    element.appendchild(script)
    return () => {
      element.removechild(script)
    }
  }, [url])
}

export default usescript

usage

usescript('/path/to/local/script.js') // async on body
usescript('https://path/to/remote/script.js', 'html') // async on html 
usescript('/path/to/local/script.js', 'html', false) // not async on html.. e.g. this will block

according to alex mcmillan's solution, i have the following adaptation.
my own environment: react 16.8+, next v9+

// add a custom component named script
// hooks/script.js

import { useeffect } from 'react'


// react-helmet don't guarantee the scripts execution order
export default function script(props) {

  // ruels: alwasy use effect at the top level and from react functions
  useeffect(() => {
    const script = document.createelement('script')

    // src, async, onload
    object.assign(script, props)

    let { parent='body' } = props

    let parentnode = document.queryselector(parent)
    parentnode.appendchild(script)

    return () => {
      parentnode.removechild(script)
    }
  } )

  return null  // return null is necessary for the moment.
}

// use the custom compoennt, just import it and substitute the old lower case <script> tag with the custom camel case <script> tag would suffice.
// index.js

import script from "../hooks/script";
    
<fragment>
  {/* google map */}
  <div ref={el => this.el = el} classname="gmap"></div>

  {/* old html script */}
  {/*<script type="text/javascript" src="http://maps.google.com/maps/api/js"></script>*/}

  {/* new custom script component */}
  <script async={false} type="text/javascript" src='http://maps.google.com/maps/api/js' />
</fragment>

you can find best answer at the following link:

https://cleverbeagle.com/blog/articles/tutorial-how-to-load-third-party-scripts-dynamically-in-javascript

const loaddynamicscript = (callback) => {
const existingscript = document.getelementbyid('scriptid');

if (!existingscript) {
    const script = document.createelement('script');
    script.src = 'url'; // url for the third-party library being loaded.
    script.id = 'libraryname'; // e.g., googlemaps or stripe
    document.body.appendchild(script);

    script.onload = () => {
      if (callback) callback();
    };
  }

  if (existingscript && callback) callback();
};

to add script tag or code in head tag <head>, use react-helmet package. it is light and have good documentation.

to add js code in script tag inside body,

    function htmldecode(html) {
      return html.replace(/&([a-z]+);/ig, (match, entity) => {
        const entities = { amp: '&', apos: '\'', gt: '>', lt: '<', nbsp: '\xa0', quot: '"' };
        entity = entity.tolowercase();
        if (entities.hasownproperty(entity)) {
          return entities[entity];
        }
        return match;
      });
    }
  render() {
    const scriptcode = `<script type="text/javascript">
          {(function() {
          window.hello={
            first_name: 'firstname',
            last_name: 'lastname',
          };
          })()}
          </script>`
    return(
      <div dangerouslysetinnerhtml={{ __html: this.htmldecode(scriptcode) }} />;
    );
  }

this code can be tested by console.log(windows.hello)

here is how i was finally able to add two external javascript files in my react js code:

these are the steps i followed.

step 1: i installed react-helmet using npm i react-helmet from the terminal while inside my react-app folder path.

step 2: i then added import {helmet} from "react-helmet"; header in my code.

step 3: finally, in my code this is how i added the external js files using helment

<helmet>
    <script src = "path/to/my/js/file1.js" type = "text/javascript" />
    <script src = "path/to/my/js/file2.js" type = "text/javascript" />  
</helmet>

you can use npm postscribe to load script in react component

postscribe('#mydiv', '<script src="https://use.typekit.net/foobar.js"></script>')

there is a very nice workaround using range.createcontextualfragment.

/**
 * like react's dangerouslysetinnerhtml, but also with js evaluation.
 * usage:
 *   <div ref={setdangeroushtml.bind(null, html)}/>
 */
function setdangeroushtml(html, el) {
    if(el === null) return;
    const range = document.createrange();
    range.selectnodecontents(el);
    range.deletecontents();
    el.appendchild(range.createcontextualfragment(html));
}

this works for arbitrary html and also retains context information such as document.currentscript.

i created a react component for this specific case: https://github.com/coreyleelarson/react-typekit

just need to pass in your typekit kit id as a prop and you're good to go.

import react from 'react';
import typekit from 'react-typekit';

const htmllayout = () => (
  <html>
    <body>
      <h1>my example react component</h1>
      <typekit kitid="abc123" />
    </body>
  </html>
);

export default htmllayout;

i tried to edit the accepted answer by @alex mcmillan but it won't let me so heres a separate answer where your able to get the value of the library your loading in. a very important distinction that people asked for and i needed for my implementation with stripe.js.

usescript.js

import { usestate, useeffect } from 'react'

export const usescript = (url, name) => {

  const [lib, setlib] = usestate({})

  useeffect(() => {
    const script = document.createelement('script')

    script.src = url
    script.async = true
    script.onload = () => setlib({ [name]: window[name] })

    document.body.appendchild(script)

    return () => {
      document.body.removechild(script)
    }
  }, [url])

  return lib

}

usage looks like

const paymentcard = (props) => {
  const { stripe } = usescript('https://js.stripe.com/v2/', 'stripe')
}

note: saving the library inside an object because often times the library is a function and react will execute the function when storing in state to check for changes -- which will break libs (like stripe) that expect to be called with specific args -- so we store that in an object to hide that from react and protect library functions from being called.

you can also use react helmet

import react from "react";
import {helmet} from "react-helmet";

class application extends react.component {
  render () {
    return (
        <div classname="application">
            <helmet>
                <meta charset="utf-8" />
                <title>my title</title>
                <link rel="canonical" href="http://example.com/example" />
                <script src="/path/to/resource.js" type="text/javascript" />
            </helmet>
            ...
        </div>
    );
  }
};

helmet takes plain html tags and outputs plain html tags. it's dead simple, and react beginner friendly.

the answer alex mcmillan provided helped me the most but didn't quite work for a more complex script tag.

i slightly tweaked his answer to come up with a solution for a long tag with various functions that was additionally already setting "src".

(for my use case the script needed to live in head which is reflected here as well):

  componentwillmount () {
      const script = document.createelement("script");

      const scripttext = document.createtextnode("complex script with functions i.e. everything that would go inside the script tags");

      script.appendchild(scripttext);
      document.head.appendchild(script);
  }

if you need to have <script> block in ssr (server-side rendering), an approach with componentdidmount will not work.

you can use react-safe library instead. the code in react will be:

import safe from "react-safe"

// in render 
<safe.script src="https://use.typekit.net/foobar.js"></safe.script>
<safe.script>{
  `try{typekit.load({ async: true });}catch(e){}`
}
</safe.script>

this answer explains the why behind this behavior.

any approach to render the script tag doesn't work as expected:

1. using the script tag for external scripts
2. using dangerouslysetinnerhtml

why

react dom (the renderer for react on web) uses createelement calls to render jsx into dom elements.

createelement uses the innerhtml dom api to finally add these to the dom (see code in react source). innerhtml does not execute script tag added as a security consideration. and this is the reason why in turn rendering script tags in react doesn't work as expected.

for how to use script tags in react check some other answers on this page.

further to the answers above you can do this:

import react from 'react';

export default class test extends react.component {
  constructor(props) {
    super(props);
  }

  componentdidmount() {
    const s = document.createelement('script');
    s.type = 'text/javascript';
    s.async = true;
    s.innerhtml = "document.write('this is output by document.write()!')";
    this.instance.appendchild(s);
  }

  render() {
    return <div ref={el => (this.instance = el)} />;
  }
}

the div is bound to this and the script is injected into it.

demo can be found on codesandbox.io

my favorite way is to use react helmet – it's a component that allows for easy manipulation of the document head in a way you're probably already used to.

e.g.

import react from "react";
import {helmet} from "react-helmet";

class application extends react.component {
  render () {
    return (
        <div classname="application">
            <helmet>
                <script src="https://use.typekit.net/foobar.js"></script>
                <script>try{typekit.load({ async: true });}catch(e){}</script>
            </helmet>
            ...
        </div>
    );
  }
};

https://github.com/nfl/react-helmet