JSFiddle - React, Tailwind, and code Playground

by Michael Darmanin

HTML

<div id="choice"></div>

<div id="hidden">
    Should return null if any of the array values are negative or more than 10. [1, -9] would return null.
</div>

CSS

#hidden {
    display:none
}

JavaScript

function upArray(arr) {
    for (var i = 0, len = arr.length; i < len; i++) {
        var num = arr[i];
        if (num > 9 || num <= 0) {
            var x = null;
            return x;
        }  
    }
    {
            var toNum = parseInt(arr.join('')) + 1;
            arr = toNum.toString().split('').map(Number);
            return arr;
        }
}

document.getElementById('choice').innerHTML = upArray([1, 3, 6, 9, -4]);