Why adding numbers in a javaScript object sorts it by default?

Question

Recently I cam across an interesting scenario while working with javaScript objects.

If we add any strings in an object it gets saved in the order that we added them

let obj = {};
obj[“z”] = true;
obj[“a”] = true;
obj[“c”] = true;

console.log(Object.keys(obj)) // [“z”,”a”,”c”]

But in the case of adding numbers, it seems to sort it by default

let obj = {};
obj[9] = true;
obj[1] = true;
obj[5] = true;

console.log(Object.keys(obj)) // [1,5,9]

What is the reason that this is happening???