JSFiddle - React, Tailwind, and code Playground

HTML

<div id="create">create</div>
<div id="concat">concat</div>

JavaScript

var arr1 = [],
    arr2;

document.getElementById('create').addEventListener('click', function() {
    for (var i = 0; i < 500000; i++) {
        arr1[i] = true;
    }
    
    console.log('created');
})

document.getElementById('concat').addEventListener('click', function() {
    arr2 = arr1;
    
    console.log('concat');
})