Script for prompt before leave form without saving

DontLeaveMeAlone.js

class DontLeaveMeAlone {
  constructor(formElem) {
    this.formElem = formElem;
    this.formCheckSum = this.getFormCheckSum();

    // Events
    this.boundBeforeunloadEventHandler = e => this.beforeunloadEventHandler(e);
    window.addEventListener('beforeunload', this.boundBeforeunloadEventHandler);

    this.formElem.addEventListener('submit', (e) => this.submitEventHandler(e))
  }

  beforeunloadEventHandler(e) {
    let currentFromCheckSum = this.getFormCheckSum();
    if (this.formCheckSum !== currentFromCheckSum) {
      e.preventDefault();
      e.returnValue = '';
    }
  }

  submitEventHandler(e) {
    window.removeEventListener('beforeunload', this.boundBeforeunloadEventHandler);
  }

  getFormCheckSum() {
    const data = new URLSearchParams(new FormData(this.formElem)).toString();

    return data;
  }
}