title: What is the use of QuestionMark(?) in typescript variable?
body: <p>I have seen a scenario where in inside interpolation QuestionMark'?' is used with variable name. 
  example : <code>current?.sample.innerSample</code></p>
  
  <p>I am not sure what it is used for. 
  Any help is much appreciated.</p>
  
  <p>Thank you</p>