express res.send as result of promise

Question

I'm not understanding what's going on...

Using q promises, this works:

const deferred = q.defer();
deferred.resolve('Hellow');

const myPromise = deferred.promise;

router.get('/items', (req, res) => {
    myPromise.then((result) => res.send(result));
});

but this doesn't, it keeps the browser like if the request never ends:

router.get('/items', (req, res) => {
    myPromise.then(res.send);
});

What's wrong?

Answer 1

Below is the fragment of the express library related to res.send:

res.send = function send(body) {
    var chunk = body;
    var encoding;
    var len;
    var req = this.req;
    var type;

    // settings
    var app = this.app;

    // allow status / body
    if (arguments.length === 2) {
        // res.send(body, status) backwards compat
        if (typeof arguments[0] !== 'number' && typeof arguments[1] === 'number') {
            deprecate('res.send(body, status): Use res.status(status).send(body) instead');
            this.statusCode = arguments[1];
        } else {
            deprecate('res.send(status, body): Use res.status(status).send(body) instead');
            this.statusCode = arguments[0];
            chunk = arguments[1];
        }
    }
 //.....

As you can see, there are lots of this references. In your case myPromise.then(res.send) the this refers to promise object, not to res, that's why your code doesn't work.

You can change the context by using .bind method, so this will refer to res object:

router.get('/items', (req, res) => {
    myPromise.then(res.send.bind(res));
});