set initial value of shortest to Number.MAX_VALUE is all you need
var myObj = {first: 45, second: 23, third: 3, fourth: 222, fifth: 2343};
var myFunc = function (object) {
var shortest = Number.MAX_VALUE;
for (var key in object) {
if (object[key] < shortest) {
shortest = object[key];
}
};
return shortest;
};
Number.MAX_VALUE The largest positive representable number.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number