React hook to fit text in a div

Date: 2019-11-15 | Tags: javascript

This is a React hook that iteratively adjusts the font size so that text will fit in a div.

checks if text is overflowing by using scrollHeight and offsetHeight from https://stackoverflow.com/a/10017343/101911
uses binary search; makes a maximum of 5 adjustments with a resolution of 5% font size from 20-100%

The code is also in a github repo: use-fit-text

const useFitText = () => {
  const MIN_FONT_SIZE = 20;
  const MAX_FONT_SIZE = 100;
  const RESOLUTION = 5;

  const ref = useRef(null);

  const [state, setState] = useState({
    fontSize: MAX_FONT_SIZE,
    fontSizePrev: MIN_FONT_SIZE,
    fontSizeMax: MAX_FONT_SIZE,
    fontSizeMin: MIN_FONT_SIZE,
  });
  const { fontSize, fontSizeMax, fontSizeMin, fontSizePrev } = state;

  useEffect(() => {
    const isDone = Math.abs(fontSize - fontSizePrev) <= RESOLUTION;
    const isOverflow =
      !!ref.current &&
      (ref.current.scrollHeight > ref.current.offsetHeight ||
        ref.current.scrollWidth > ref.current.offsetWidth);
    const isAsc = fontSize > fontSizePrev;

    // return if the font size has been adjusted "enough" (change within RESOLUTION)
    // reduce font size by one increment if it's overflowing
    if (isDone) {
      if (isOverflow) {
        const fontSizeNew =
          fontSizePrev < fontSize
            ? fontSizePrev
            : fontSize - (fontSizePrev - fontSize);
        setState({
          fontSize: fontSizeNew,
          fontSizeMax,
          fontSizeMin,
          fontSizePrev,
        });
      }
      return;
    }

    // binary search to adjust font size
    let delta;
    let newMax = fontSizeMax;
    let newMin = fontSizeMin;
    if (isOverflow) {
      delta = isAsc ? fontSizePrev - fontSize : fontSizeMin - fontSize;
      newMax = Math.min(fontSizeMax, fontSize);
    } else {
      delta = isAsc ? fontSizeMax - fontSize : fontSizePrev - fontSize;
      newMin = Math.max(fontSizeMin, fontSize);
    }
    setState({
      fontSize: fontSize + delta / 2,
      fontSizeMax: newMax,
      fontSizeMin: newMin,
      fontSizePrev: fontSize,
    });
  }, [fontSize, fontSizeMax, fontSizeMin, fontSizePrev, ref]);

  return { fontSize: `${fontSize}%`, ref };
};

Example usage¶

import React from "react";
import useFitText from "use-fit-text";

const Example = () => {
  const { fontSize, ref } = useFitText();

  return (
    <div ref={ref} style={{ fontSize, height: 40, width: 100 }}>
      Lorem ipsum dolor sit amet, consectetur adipiscing elit.
    </div>
  );
}