getChanges.js

function getChanges(oldText, newText) {
  let index

  for (let i in oldText) {
    let charOld = oldText[i]
    let charNew = newText[i]

    if (charOld !== charNew) {
      index = i
      break
    }
  }

  if (index === undefined) {
    return {
      content: newText.substr(oldText.length),
      removeTimes: 0
    }
  }

  return {
    content: newText.substr(index),
    removeTimes: oldText.length - index
  }
}