JSFiddle - React, Tailwind, and code Playground

JavaScript

var globalData = {
    arr: [10, 20]
};

var handleData = function(data) {
    var privateData = data;
    privateData.arr.shift();
    privateData.arr.push(30);
}

console.log(globalData.arr);
handleData(globalData);
console.log(globalData.arr);