JavaScript 'in' operator for `undefined` elements in Arrays

Question

Please consider the following snippet of code:

> a = [1, undefined, undefined, undefined, 3]
  [1, undefined, undefined, undefined, 3]
> b = [1,,,,3]
          


        

Answer 1

//-1 means not in the list
//['a','b','c'].indexOf('a') is return index if not find in the list then it give -1.

if(['a','b','c'].indexOf('d') == -1)
{
    console.log('yes it -1')
}