JSFiddle - React, Tailwind, and code Playground

JavaScript

var s = '1,2,,,3,4,,,,,,5,6';
var a = s.split(',');
for (var i = 0; i < a.length; i++) {
    if (a[i] == "") {
        a[i] = "null";
    }
}
document.body.innerHTML = '[' + a.join(',') + ']';