JSFiddle - React, Tailwind, and code Playground

HTML

<div id="result"></div>

JavaScript

function copyByReference(arr) {
    return {
        data: arr
    };
}

function copyByValue(arr) {
    return {
        data: arr.slice(0)
    };
}


var arr = [1,2,3]
var obj1 = copyByReference(arr);
var obj2 = copyByValue(arr);

arr.push(4);

$('#result').html('obj1.data.length: ' + obj1.data.length + ', obj2.data.length: ' + obj2.data.length);