A simple affix React component.

AffixWrapper.jsx

/**
 * @author Juliano Castilho <julianocomg@gmail.com>
 */
var React = require('react');

var AffixWrapper = React.createClass({
  /**
   * @type {Object}
   */
  propTypes: {
    offset: React.PropTypes.number
  },

  /**
   * @return {Object}
   */
  getDefaultProps() {
    return {
      offset: 0
    };
  },

  /**
   * @return {Object}
   */
  getInitialState() {
    return {
      affix: false
    };
  },

  /**
   * @return {void}
   */
  handleScroll() {
    var affix = this.state.affix;
    var offset = this.props.offset;
    var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;

    if (!affix && scrollTop >= offset) {
      this.setState({
        affix: true
      });
    }

    if (affix && scrollTop < offset) {
      this.setState({
        affix: false
      });
    }
  },

  /**
   * @return {void}
   */
  componentDidMount() {
    window.addEventListener('scroll', this.handleScroll);
  },

  /**
   * @return {void}
   */
  componentWillUnmount() {
    window.removeEventListener('scroll', this.handleScroll);
  },

  render() {
    var affix = this.state.affix ? 'affix' : '';
    var {className, offset, ...props} = this.props;

    return (
      <div {...props} className={className + ' ' + affix)}>
        {this.props.children}
      </div>
    );
  }

});

module.exports = AffixWrapper;

Example.jsx

<AffixWrapper className="some-cool-element" id="lalala" offset={200}>
  <div>Put whatever you want here</div>
</AffixWrapper>

A functional component with react hook alternative that preserve the width

import React from 'react';

export default function Affix(props: {
  top: number;
  children: React.ReactNode;
  offset?: number;
  className?: string;
}) {
  const element = React.createRef<HTMLDivElement>();
  let oldStyles = {
    position: '',
    top: '',
    width: '',
  };
  // Offset could make the element fixed ealier or later
  const { offset = 0 } = props;

  const checkPosition = (distanceToBody: number, width: number) => {
    const scrollTop = window.scrollY;

    if (distanceToBody - scrollTop < props.top + offset) {
      if (element.current.style.position != 'fixed') {
        for (let key in oldStyles) {
          oldStyles[key] = element.current.style[key];
        }
        element.current.style.position = 'fixed';
        element.current.style.width = width + 'px';
        element.current.style.top = props.top + 'px';
      }
    } else {
      // reset to default
      for (let key in oldStyles) {
        element.current.style[key] = oldStyles[key];
      }
    }
  };

  React.useEffect(() => {
    if (typeof window.scrollY === 'undefined') {
      // don't work in IE
      return;
    }

    const distanceToBody = window.scrollY + element.current.getBoundingClientRect().top;
    const handleScroll = () => {
      requestAnimationFrame(() => {
        checkPosition(distanceToBody, element.current.clientWidth);
      });
    };

    window.addEventListener('scroll', handleScroll);
    return () => {
      window.removeEventListener('scroll', handleScroll);
    };
  });

  return (
    <div ref={element} style={{ zIndex: 1 }} className={props.className}>
      {props.children}
    </div>
  );
}