JSFiddle - React, Tailwind, and code Playground

JavaScript

var foo = "IsPreviewed:'1',PreviewdUrl:'www.google.co.uk',IsFreeVideo:'y'";

var variable = "PreviewdUrl";
var startIndex = foo.indexOf(variable) + variable.length + 2;
var endIndex = foo.indexOf(",", startIndex) - 1;

alert(foo.substring(startIndex, endIndex));