JSFiddle - React, Tailwind, and code Playground

HTML

<input id="userDateInput" type="date" onchange="findDates()"> <br>

<strong for="firstDay">First Day:</strong> <span id="firstDay"></span> <br>
<strong for="lastDay">Last Day:</strong> <span id="lastDay"></span> <br>

JavaScript

function findDates() {
	const userInput = document.getElementById('userDateInput').value;
  const dates = getFirstLastDayOfWeek(userInput);
  
  document.getElementById('firstDay').innerHTML = dates.firstDay;
  document.getElementById('lastDay').innerHTML = dates.lastDay;
}

function getFirstLastDayOfWeek(userDate) {
	
  let result = {};

  var curr = new Date(userDate); // get current date
  var first = curr.getDate() - curr.getDay(); // First day is the day of the month - the day of the week
  var last = first + 6; // last day is the first day + 6
  
  result = {
		firstDay:new Date(curr.setDate(first)).toUTCString(),
    lastDay:new Date(curr.setDate(last)).toUTCString()
  };
  
  return result;
};