How to find missing characters from string B based on string A and add them to string B? : I have 2 strings.

    var stringA = "LetDoItNow"
    var stringB = "LetDo"
Is there a way that I can find missing characters from string B based on string A and add them to it?

The missing characters in this case are "ItNow" and my expected result after checking is

    stringB = "LetDoItNow"

Thank you so much!