A string with values vs an array of values

A file I'm working with that was made long before I started working here has a list of 53 different numeric values. But it has them all listed as a string separated by commas as such for example:

var numbers = "1,2,3,4,5,...,48,49,50";

Later in the script, when there is a check to see if a variable that's passed in exists, it uses:

if(numbers.indexOf(String(x),0) < 0) {/*do stuff*/} else {/*do other stuff*/}

While this works, I can't help but wonder why this way of doing it was chosen as opposed to making it an array of numbers. Would that be more optimal in terms of run-time then doing it the way it is currently set up?