If you’re building an RESTfull API with NodeJS (and Express), you’ll probably need CORS.
Otherwise nobody will be able to user your public API via Browsers.
So this is important vor Third-Party JQuery, Angular, whatever… Clients.

With this small Piece of Code you could make your API public and answer the CORS (Preflight) Requests.

This small function can be used via:
var express = require('express');
var app = express();
app.use(allowCrossDomain);