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

Question

I am using Express v3.4.4. When I try to do like this:

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

I get an error message:

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

In code, worker:

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

does not work:

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

Answer 1

send actually a function of the res object. He tries to use other data from the object res . But when you do

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

you simply use the send function object without reference to the res object. That is why it does not work.

---

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

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

Now

 cb({...}); 

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

The bind function is actually Function.prototype.bind

Answer 2

This is because in JavaScript, the value of this inside the method is bound to the contained object.

• The call to res.send() makes this value inside the send res function.
• In contrast, when invoking an unrelated version of cb() , this not expected.

The internal implementation of Express.js is based on the fact that this is related and most likely contains an expression such as this.method that raises a TypeError: Cannot read property 'method' of undefined error.