CHAP05.md

In this chapter, we will be implementing simple res.send method.

Actually, res.send in express does many things like the following:

1. setting headers
2. content types
3. Converting buffer to right datatype.
4. Setting up statuscode etc.

We wont be handling all the cases in our implementation, because they are just wrappers over core response methods.

The full source of the res.send is over here: https://github.com/expressjs/express/blob/master/lib/response.js#L107.Actually many useful methods that we as developers use in res object is extended here.

We talked about extending the core response object to add useful methods in chapter 4 and we did have a dirty send method like the following:

res.send = function (body) {
    console.log("wow,", body)
}

Okay, lets go and make changes so that we can send string and JSON to our little send method.

This is what we need to do:

1. Set Headers using core setHeader method.
2. Set Content-type to text/plain for strings and application/json for JSON
3. Flush out the content to our response object.

Simple right? The implementation looks like the following:

res.send = function (body) {
    if(typeof body === 'object') {
        this.setHeader('Content-Type', 'application/json');
        this.end(JSON.stringify(body),'utf8');
    }
    else if(typeof body === 'string') {
        this.setHeader('Content-Type', 'text/plain');
        this.end(body,'utf8');
    }
    return this;
}

Now lets run our code from index.js:

let express = require('./src/express')
const app = express()

app.get('/', (req, res,next) => {
    console.log(next)
    next()
});


app.get('/', (req, res) => {
    res.writeHead(200)
    res.write('Response from second matching route');
    res.send("hello world")
    res.end();
});

app.post('/post',(req,res) => {
    res.writeHead(200)
    res.write('Data from post :)');
    res.end();
})


app.listen(3000, () => console.log('Example app listening on port 3000!'))

We would get the following error on the console:

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at validateHeader (_http_outgoing.js:503:11)
    at ServerResponse.setHeader (_http_outgoing.js:510:3)
    at ServerResponse.res.send (/Users/abelginrayen/programs/js/lets-build-express/chap05/src/express.js:23:18)
    at app.get (/Users/abelginrayen/programs/js/lets-build-express/chap05/index.js:13:9)
    at Layer.handle [as handle_request] (/Users/abelginrayen/programs/js/lets-build-express/chap05/src/Layer.js:29:9)
    at next (/Users/abelginrayen/programs/js/lets-build-express/chap05/src/router.js:74:28)
    at expressInit (/Users/abelginrayen/programs/js/lets-build-express/chap05/src/middleware/init.js:6:9)
    at Layer.handle [as handle_request] (/Users/abelginrayen/programs/js/lets-build-express/chap05/src/Layer.js:29:9)
    at next (/Users/abelginrayen/programs/js/lets-build-express/chap05/src/router.js:78:19)
    at app.get (/Users/abelginrayen/programs/js/lets-build-express/chap05/index.js:6:5)

Why? The reason is simple and by now should make sense, in our little send method, we actually do set headers via setHeaders and also do call end function. However the above code do have writeHead in it. Let's remove them:

app.get('/', (req, res) => {
    res.send("hello world")
});

Note: This is the same reason, we shouldn't call res.send twice. Because internally we call end and close

the response object.

Now we should be getting response as hello world. Now lets try with an json like:

app.get('/', (req, res) => {
    res.send({hello: 'world'})
});

now the response would be: {"hello":"world"}. Great.

In fact, we can have a function json on response object too:

res.send = function (body) {
    if(typeof body === 'object') {
        this.json(body)
    }
    else if(typeof body === 'string') {
        this.setHeader('Content-Type', 'text/plain');
        this.end(body,'utf8');
    }
    return this;
}


res.json = function (body) {
    this.setHeader('Content-Type', 'application/json');
    return this.send(JSON.stringify(body))
}

Now we can do:

app.get('/', (req, res) => {
    res.json({hello: 'world'})
});

to get the same results.

We have now useful functions on response objects. But still we haven't implemented error handling our little express.

We will discuss about error handling on next chapter.