JSFiddle - React, Tailwind, and code Playground

JavaScript

var ars = [1, [2, [3, null]]], rev = null;

function r(x) {
    rev = (rev == null) ? [x[0]] : [x[0], rev];
    if( x[1] !== null )
        r(x[1]);
}
r(ars);
console.log( rev );