var installPromptEvnt = null;

window.onload = () => {
    window.addEventListener("beforeinstallprompt", (e) => {
        installPromptEvnt = e;
        document.getElementById("inp_bttn").disabled = false;
    });

    document.getElementById("inp_bttn").addEventListener("click", function(){
        installPromptEvnt.prompt();
        this.disabled = true;
    });
};