JS Bin// source https://jsbin.com/nayedi

diff.js

function findDiff(original, newText) {

  function getZip(originalList, newTextList){
    return originalList.map(function(char,i) {
      return {
        first: char,
        second: newTextList[i]
      }
    })
  }

  function findDiffPoint(zippedList) {
    return zippedList.map(function(pair) {
      return (pair.first == pair.second)
    }).indexOf(false)
  }
  
  var zip = getZip(
    original.split(''), 
    newText.split('')
  )
  var diffStart = findDiffPoint(zip)
  var reverseZip = getZip(
    original.split('').reverse(), 
    newText.split('').reverse()
  )
  
  var diffEnd = newText.length - findDiffPoint(reverseZip)
  
  var added = newText.slice(diffStart,diffEnd)
  
  return {
    start: diffStart,
    end: diffEnd,
    added: added
  }
}


var original = "first para\n Second para"
var newText =  "first para\n <b> INSERT </b> Second para"

diff = findDiff(original,newText)
console.log(
  diff
)

output.md

[object Object] {
  added: "<b> INSERT </b>",
  end: 27,
  start: 12
}