singleton + expressjs + javascript

gistfile1.js

var constants = require('../config/constants');
var url = constants.APP_URL;
var soap = require('soap');

var singleton = function singleton() {
    var instance = null;
    soap.createClient(url, function(err, client) {
        if (err) {
            console.log(err);
            instance = null;
        } else {
            instance = client;
        }
    });
    this.getClientInstance = function() {
        return instance;
    };
    if (singleton.caller != singleton.getInstance) {
        throw new Error("This object cannot be instanciated");
    }
}
singleton.instance = null;
singleton.getInstance = function() {
    if (this.instance === null) {
        this.instance = new singleton();
    }
    return this.instance;
}
module.exports = singleton.getInstance();