JSFiddle - React, Tailwind, and code Playground

JavaScript

var boxObj = [];
var obj = {
    id : 1,
    obj_id : 234
}

boxObj.push(obj);

if (localStorage.getItem('boxObj') == null) {
    localStorage.setItem('boxObj', JSON.stringify(boxObj));
} else {
   var obj_2 = [];
   var obj_1 = [];
   obj_1 = JSON.parse(localStorage.getItem('boxObj'));
   obj_2 = obj_1.concat(boxObj);
   console.log(obj_2);
   localStorage.setItem('boxObj', JSON.stringify(obj_2));
}