Completely flew over my head to use ternary operators here!

Conditional operator

What does the ? and : mean in Javascript?

The second part (true or false) is unnecesary. It's already assesed in the parentheses

-1, needs more ?:.

?

I see this sort of thing a lot in code, and it's redundant. You should just return the original conditional check. Consider it as if it were the folowing:

if (isTrue)
  return true;
else
  return false;

The above would be better written:

return isTrue;

Same idea with what is written in this solution.

It's clear though. It's a suggestion, not an issue ;-)

Everything looks good, could be improved by adding a few test cases

Could do with a better explanation of what is meant by 'lost number'

As a beginner.. not impressed.. way too easy. And the instructions are too open. A lot of room for improvement this teaches nothing and I learnt nothing from it.

As it stands the current kata is way too easy. It needs some test cases that will make the user think and also needs to have a better description

Needs a better description rather than just the usage

Need to provide feedback as to what the values tested are when a test case fails