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

The example below uses react hook useEffect.

Working example here

import React, { useRef, useLayoutEffect, useState } from "react";

const ComponentWithDimensions = props => {
  const targetRef = useRef();
  const [dimensions, setDimensions] = useState({ width:0, height: 0 });

  useLayoutEffect(() => {
    if (targetRef.current) {
      setDimensions({
        width: targetRef.current.offsetWidth,
        height: targetRef.current.offsetHeight
      });
    }
  }, []);

  return (
    <div ref={targetRef}>
      <p>{dimensions.width}</p>
      <p>{dimensions.height}</p>
    </div>
  );
};

export default ComponentWithDimensions;

Some Caveats

useEffect will not be able to detect it's own influence to width and height

For example if you change the state hook without specifying initial values (eg const [dimensions, setDimensions] = useState({});), the height would read as zero when rendered because

• no explicit height was set on the component via css
• only content drawn before useEffect can be used to measure width and height
• The only component contents are p tags with the height and width variables, when empty will give the component a height of zero
• useEffect will not fire again after setting the new state variables.

This is probably not an issue in most use cases, but I thought I would include it because it has implications for window resizing.

Window Resizing

I also think there are some unexplored implications in the original question. I ran into the issue of window resizing for dynamically drawn components such as charts.

I'm including this answer even though it wasn't specified because

1. It's fair to assume that if the dimensions are needed by the application, they will probably be needed on window resize.
2. Only changes to state or props will cause a redraw, so a window resize listener is also needed to monitor changes to the dimensions
3. There's a performance hit if you redraw the component on every window resize event with more complex components. I found introducing setTimeout and clearInterval helped. My component included a chart, so my CPU spiked and the browser started to crawl. The solution below fixed this for me.

code below, working example here

import React, { useRef, useLayoutEffect, useState } from 'react';

const ComponentWithDimensions = (props) => {
    const targetRef = useRef();
    const [dimensions, setDimensions] = useState({});

    // holds the timer for setTimeout and clearInterval
    let movement_timer = null;

    // the number of ms the window size must stay the same size before the
    // dimension state variable is reset
    const RESET_TIMEOUT = 100;

    const test_dimensions = () => {
      // For some reason targetRef.current.getBoundingClientRect was not available
      // I found this worked for me, but unfortunately I can't find the
      // documentation to explain this experience
      if (targetRef.current) {
        setDimensions({
          width: targetRef.current.offsetWidth,
          height: targetRef.current.offsetHeight
        });
      }
    }

    // This sets the dimensions on the first render
    useLayoutEffect(() => {
      test_dimensions();
    }, []);

    // every time the window is resized, the timer is cleared and set again
    // the net effect is the component will only reset after the window size
    // is at rest for the duration set in RESET_TIMEOUT.  This prevents rapid
    // redrawing of the component for more complex components such as charts
    window.addEventListener('resize', ()=>{
      clearInterval(movement_timer);
      movement_timer = setTimeout(test_dimensions, RESET_TIMEOUT);
    });

    return (
      <div ref={ targetRef }>
        <p>{ dimensions.width }</p>
        <p>{ dimensions.height }</p>
      </div>
    );
}

export default ComponentWithDimensions;

re: window resizing timeout - In my case I'm drawing a dashboard with charts downstream from these values and I found 100ms on RESET_TIMEOUT seemed to strike a good balance for me between CPU usage and responsiveness. I have no objective data on what's ideal, so I made this a variable.

As it was already mentioned, you can't get any element's dimensions until it is rendered to DOM. What you can do in React is to render only a container element, then get it's size in componentDidMount, and then render rest of the content.

I made a working example.

Please note that using setState in componentDidMount is an anti-pattern but in this case is fine, as it is exactly what are we trying to achieve.

Cheers!

Code:

import React, { Component } from 'react';

export default class Example extends Component {
  state = {
    dimensions: null,
  };

  componentDidMount() {
    this.setState({
      dimensions: {
        width: this.container.offsetWidth,
        height: this.container.offsetHeight,
      },
    });
  }

  renderContent() {
    const { dimensions } = this.state;

    return (
      <div>
        width: {dimensions.width}
        <br />
        height: {dimensions.height}
      </div>
    );
  }

  render() {
    const { dimensions } = this.state;

    return (
      <div className="Hello" ref={el => (this.container = el)}>
        {dimensions && this.renderContent()}
      </div>
    );
  }
}

You cannot. Not reliably, anyway. This is a limitation of browser behavior in general, not React.

When you call $('#container').width(), you are querying the width of an element that has rendered in the DOM. Even in jQuery you can't get around this.

If you absolutely need an element's width before it renders, you will need to estimate it. If you need to measure before being visible you can do so while applying visibility: hidden, or render it somewhere discretely on the page then moving it after measurement.