NodeJS Express. res.send() fails when assigned to another var

Question

I'm using Express v3.4.4. When i try to do like this:

var cb = res.send;
cb(result);

I get an error:

   ...\node_modules\express\lib\response.js:84
   var HEAD = 'HEAD' == req.method;
   TypeError: Cannot read property 'method' of undefined

In code, working one:

    workflow.on('someEvent', function () {
      res.send({
          error: null,
          result: 'Result'
        });
    }); 

not working:

    workflow.on('someEvent', function () {
      var cb = res.send; 
      cb({
          error: null,
          result: 'Result'
      });
    });    

Answer 1

The send is actually a function of the object res. It tries to use other data from the res object. But, when you do

var cb = res.send;
cb({...});

you are simply using the function object send without the reference to the res object. That is why it is not working.

---

If you ever need to do something like that, then bind the res object to the send function like this

var cb = res.send.bind(res);

now

cb({...});

will also work. Because the res is bound to the send function object and the resulting function is stored in cb.

The bind function actually is Function.prototype.bind