JSFiddle - React, Tailwind, and code Playground

JavaScript

// Form Data String
var dataString = "sendNotification=true&type=extended&additionalNotes=&returnToMainPage=true";

// Split the String using String.split() method. It will return the array.
var params = dataString.split("&");

// Create the destination object.
var obj = {};

// iterate the splitted String and assign the key and values into the obj.
for (var i in params) {
  var keys = params[i].split("=");
  obj[keys[0]] = keys[1];
}

console.log(obj); // Object {sendNotification: "true", type: "extended", additionalNotes: "", returnToMainPage: "true"}