React: How to wait until ref is available when inserted (rendered) within a second container

Question

EDIT: better explanation

The context:

I receive some plain HTML code from a 3rd server, which I want to

• insert in my React app
• modify it

The vanilla JS approach

• I can modify the string with regex and add any HTML tag with an id
• Then I can modify these elements through getElementById, as usual

The React approach

• I shouldn't use the DOM
• Then I should insert within the string some components that have a React ref inside
• The opposite (to insert some React components as plain HTML) would be through ReactDOMServer.renderToString
• So, when I inject the components with ReactDOM.render(), the problem is that the render method takes its time, so that if in the next line I try to use the ref that exists in the inserted component, is not yet there

The question

• How to do it? Usually I would put the code within a useEffect with a [] dependencies, but here I am rendering the component when the app is already mounted
• A quick workaround is to just do an async wait of 500 ms, and then I can access the ref, but for sure there has to be something better

---

This code fails, because when the ref is rendered it is still not available, so ref.current is undefined

How can I wait for it?

codesandbox

EDIT: I provide the code that works but through direct DOM, which I assume should be avoided

import React, { useRef, useEffect } from "react";
import ReactDOM from "react-dom";

export default function App() {
  const myref = useRef();

  useEffect(() => {
    const Com = () => <div ref={myref}>hello</div>;
    ReactDOM.render(<Com />, document.getElementById("container"));
    console.log(myref.current); // undefined
    document.getElementById('container').textContent = "direct DOM works"

   // the next line fails since the ref is not yet available
   // myref.current.textContent = "but this REF is not available"; // fails
  }, []);

  const plainhtml = '<div><div id="container"></div><div>some more content</div><div id="another">even more content</div></div>'; // this is some large HTML fetched from an external server

  return (
    <div>
      <h1>Hello CodeSandbox</h1>
      <div dangerouslySetInnerHTML={{ __html: plainhtml }} />
    </div>
  );
}

Answer 1

useEffect with empty dependency array executes after the first render, therefore you will get the DOM ref in the callback:

const htmlString = '<div id="container">Hello</div>';

export default function App() {
  const myRef = useRef();

  useEffect(() => {
    if (myRef.current) {
      myRef.current.textContent = 'whats up';
    }
    console.log(myRef.current);
  }, []);

  return (
    <div>
      <div ref={myRef} dangerouslySetInnerHTML={{ __html: htmlString }} />
      <div dangerouslySetInnerHTML={{ __html: htmlString }} />
    </div>
  );
}

/* App renders:
whats up
Hello
*/

Answer 2

I need to use a callback ref but encapsulating it within useCallback to make sure it only rerenders with the dependencies indicated (i.e. none []), so that it is only executed when the component changes (as explained here)

codesandbox

import React, { useEffect, useCallback } from "react";
import ReactDOM from "react-dom";

export default function App() {
  const measuredRef = useCallback(node => {
    if (node !== null) {
      node.textContent = "useCallback DOM also works";
    }
  }, []);

  useEffect(() => {
    const Com = () => <div ref={measuredRef}>hello</div>;
    ReactDOM.render(<Com />, document.getElementById("container"));
    document.getElementById("container").textContent = "direct DOM works";
  }, []);

  const plainhtml = '<div id="container"></div>';

  return (
    <div>
      <h1>Hello CodeSandbox</h1>
      <div dangerouslySetInnerHTML={{ __html: plainhtml }} />
    </div>
  );
}