How to use if statement to declare a new variable with ES6? JavaScript

How can I optionally declare a variable using let or const? For example it would be possible to do:

if (variableNotMade) {
  var makeVariable
}

But how can this be achieved using let or const instead of var?
A similar post here shows that it's possible to do:

let makeVariable = variableNotMade ? foo : bar

Although this approach only really works if you have an else.
Is there a suitable way to achieve this or should a different approach be taken?