How to get a react component's size (height/width) before render?

Question

I have a react component that needs to know its dimensions ahead of time, before it renders itself.

When I\'d make a widget in jquery I could just <

Answer 1

There's an unexpected "gotcha" with @shane's approach for handling window resizing: The functional component adds a new event listener on every re-render, and never removes an event listener, so the number of event listeners grows exponentially with each resize. You can see that by logging each call to window.addEventListener:

window.addEventListener("resize", () => {
  console.log(`Resize: ${dimensions.width} x ${dimensions.height}`);
  clearInterval(movement_timer);
  movement_timer = setTimeout(test_dimensions, RESET_TIMEOUT);
});

This could be fixed by using an event cleanup pattern. Here's some code that's a blend of @shane's code and this tutorial, with the resizing logic in a custom hook:

/* eslint-disable react-hooks/exhaustive-deps */
import React, { useState, useEffect, useLayoutEffect, useRef } from "react";

// Usage
function App() {
  const targetRef = useRef();
  const size = useDimensions(targetRef);

  return (
    

      
{size.width}
      
{size.height}
    
  );
}

// Hook
function useDimensions(targetRef) {
  const getDimensions = () => {
    return {
      width: targetRef.current ? targetRef.current.offsetWidth : 0,
      height: targetRef.current ? targetRef.current.offsetHeight : 0
    };
  };

  const [dimensions, setDimensions] = useState(getDimensions);

  const handleResize = () => {
    setDimensions(getDimensions());
  };

  useEffect(() => {
    window.addEventListener("resize", handleResize);
    return () => window.removeEventListener("resize", handleResize);
  }, []);

  useLayoutEffect(() => {
    handleResize();
  }, []);
  return dimensions;
}

export default App;

There's a working example here.

This code doesn't use a timer, for simplicity, but that approach is further discussed in the linked tutorial.