Why does setting the condition of an variable to be undefined give an error?

I want the function to run only once on the first click and no more. But I get an error that someVar is not defined, that is what I have included in the condition. Why is it giving the error? How to solve it?

button.onclick = function () {
  if (someVar == undefined) {
    console.log("Hi");
    someVar = 1; 
  }
}