array_num.js

function listNum(operation) {
    let oneNumber = parseInt(this.join(''))
    return operation(oneNumber).toString()
        .split('')
        .map(val => new Number(val))
}

Array.prototype.increment = function() {
    return listNum.call(this, (val) => ++val)
}

Array.prototype.decrement = function() {
    return listNum.call(this, (val) => --val)
}