JSFiddle - React, Tailwind, and code Playground

HTML

<iframe id="dlFrame" onload="BlockAndSpin(false);"></iframe>

<button onclick="downloadPOS('dlFrame')">load</button>

JavaScript

function downloadPOS(id) {
    var ifrm = document.getElementById(id)
    BlockAndSpin(true);
    ifrm.onload = function () {
        BlockAndSpin(false);
    }
}

function BlockAndSpin(flag) {
    if (!flag) {
        alert('!');
    }
}