[рони]

Math.abs

это обязательно?

let array = [1,2,3,4,5];
let min = array.at(0);
let max = array.at(-1);
for (let num of array) {
    if (num < min) {
        min = num;
    }
    else if (num > max) {
        max = num;
    }
}

const difference = max - min;
console.log(difference)