Arguments are a mere suggestion to call a function in JavaScript. If you supply more arguments than required, they are simply ignored. If you supply less, the missing ones are set to undefined.
To handle the case of fewer arguments than the function specified, you can either check for undefined or provide default arguments.
Both of these work:
const avg = (x, y) => y === undefined ? x : (x + y) / 2;
const avg = (x, y = x) => (x + y) / 2;
I prefer the latter but your mileage may vary.