Bootstrap alers ES6 singleton class

Alert.js

import $ from 'jquery';
import domready from 'domready';
import is from 'is';

class Alert{
    constructor(alertsContainer) {

        if(!Alert.instance){
            Alert.instance = this;
        }else{
            return Alert.instance;
        }

        this.container = alertsContainer;

        return Alert.instance;
    }

    pushNewAlert(type,msg){
        let html = Alert.toAlertString(type,msg);
        $(this.container).append(html);
    }

    static toAlertString(type,msg){
        return `
<div class="alert alert-${type} alert-dismissible fade in" role="alert">
    <button type="button" class="close" data-dismiss="alert">
    <span data-aria-hidden="true">&times;</span>
    <span class="sr-only">${Translator.trans('front.close')}</span>
</button>
${msg}
</div>`;
    }
}

Alert.instance = null;

Alert.TYPE = {
    WARN: 'warning',
    ERR: 'danger',
    INFO: 'info',
    OK: 'success'
};

domready(function () {
    let alertsContainer = document.getElementById("flash-messages");
    if (!is.nil(alertsContainer) && is.defined(alertsContainer)) {
        new Alert(alertsContainer);
    }
});


export default Alert;