React: Prevent scroll when modal is open

in functional component for when mount or unmount

  useEffect(() => {
     document.body.style.overflow = 'hidden';
     return ()=> document.body.style.overflow = 'unset';
  }, []);

or when show is change or etc

  useEffect(() => {
     show && document.body.style.overflow = 'hidden';
     !show && document.body.style.overflow = 'unset';
  }, [show ]);

In functional components if the browser move due to hiding/showing scroll.

  useEffect(() => {
    if (show) {
      document.body.style.overflow = 'hidden';
      document.body.style.paddingRight = '15px';
    }
    return () => {
      document.body.style.overflow = 'unset';
      document.body.style.paddingRight = '0px';
    };
  }, [show]);

Use state to track if the Modal is open and only hide scroll if it's true. Since you're using document.body.style.overflow = 'hidden' in componentDidMount, the component still gets mounted which calls the lifecycle method that hides the scroll on body.

export class Modal extends React.Component {

constructor(props) {
    super(props);
    this.state = {
      open:false
    }
}

componentDidMount() {    
  if(this.state.open){
    document.body.style.overflow = 'hidden';
  }    
}

componentWillUnmount() {
    document.body.style.overflow = 'unset';
}

render() {
    return (
        <React.Fragment>
            {this.props.showAt ?
                this.props.show ?
                    <div style={style} className={`${this.props.sectionName} ${modalTypeStyle ? modalTypeStyle : styles.modalWhite} ${modalTypeSize ? modalTypeSize : styles.modalSmall} ${!this.props.showAt ? styles.modalWhiteFixed : ""}`}>
                        {this.props.arrowShape ? <div className={arrowTypeStyle ? arrowTypeStyle : styles.triangleToprightWhite} /> : null}
                        {this.props.children}
                    </div>
                    : null
                :
                this.props.show ?
                    <div className={`${this.props.className} ${styles.modal}`}>
                        <div style={style} className={`${this.props.sectionName} ${modalTypeStyle ? modalTypeStyle : styles.modalWhite} ${modalTypeSize ? modalTypeSize : styles.modalSmall} ${!this.props.showAt ? styles.modalWhiteFixed : ""}`}>
                            {this.props.arrowShape ? <div className={arrowTypeStyle ? arrowTypeStyle : styles.triangleToprightWhite} /> : null}
                            {this.props.children}
                        </div>
                    </div> :
                    null}
        </React.Fragment>
    )
  }
}

Here is a custom hook you can use for functional components.

import { useEffect } from 'react';

export const useDisableBodyScroll = (open) => {
  useEffect(() => {
    if (open) {
      document.body.style.overflow = 'hidden';
    } else {
      document.body.style.overflow = 'unset';
    }
  }, [open]);
};

export const Modal = (props) => {
    useDisableBodyScroll(props.show);

    return (
        <React.Fragment>
            {props.showAt ?
                props.show ?
                    <div style={style} className={`${props.sectionName} ${modalTypeStyle ? modalTypeStyle : styles.modalWhite} ${modalTypeSize ? modalTypeSize : styles.modalSmall} ${!props.showAt ? styles.modalWhiteFixed : ""}`}>
                        {props.arrowShape ? <div className={arrowTypeStyle ? arrowTypeStyle : styles.triangleToprightWhite} /> : null}
                        {props.children}
                    </div>
                    : null
                :
                props.show ?
                    <div className={`${props.className} ${styles.modal}`}>
                        <div style={style} className={`${props.sectionName} ${modalTypeStyle ? modalTypeStyle : styles.modalWhite} ${modalTypeSize ? modalTypeSize : styles.modalSmall} ${!props.showAt ? styles.modalWhiteFixed : ""}`}>
                            {props.arrowShape ? <div className={arrowTypeStyle ? arrowTypeStyle : styles.triangleToprightWhite} /> : null}
                            {props.children}
                        </div>
                    </div> :
                    null}
        </React.Fragment>
    )
  }
}