nodejs function hoisting : why it doesn't work?

Question

This works:

    var http = require('http');

    var handler = function (req, res) {
        res.writeHead(200, {'Content-Type': 'text/plain'});
        res.end('Hello World!');
    }


    http.createServer(handler).listen(8080);

But this doesn't

    var http = require('http');

    http.createServer(handler).listen(8080);

    var handler = function (req, res) {
        res.writeHead(200, {'Content-Type': 'text/plain'});
        res.end('Hello World!');
    }

I don't understand why since it should with hoisting all the more I got no error.

Because you haven't defined handler yet? It only passes variables by reference when it's an object — lumio, Oct 17 '17 at 12:26
@lumio hoisting normally allows to define var afterwards so my question. — user310291, Oct 17 '17 at 12:27
Hoisting only applies to function declarations and not function expressions: https://stackoverflow.com/q/336859/1169798 — Sirko, Oct 17 '17 at 12:28
@user310291 but the way you wrote it, is that you define your function after you pass it. That might be the problem here — lumio, Oct 17 '17 at 12:31
@user310291 You're using a function expression and not a declaration here. Maybe also see https://stackoverflow.com/q/1013385/1169798 — Sirko, Oct 17 '17 at 12:31
There is no function hoisting for a function expression `var handler = function() {}`, only with function definition `function handler() {}`. In your function expression, only the variable itself is hoisted, not the assignment to it. — jfriend00, Oct 17 '17 at 12:41

score 12 · Accepted Answer · answered Oct 17 '17 at 12:29

That's not function hoisting, that's variable hoisting. It's equivalent to this:

var http = require('http');
var handler;

http.createServer(handler).listen(8080);

handler = function (req, res) {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello World!');
}

Function hoisting works only for function declarations (the above is a function expression):

var http = require('http');

http.createServer(handler).listen(8080);

function handler(req, res) {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello World!');
}

More info: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function#Function_declaration_hoisting

score 1 · Answer 2 · edited Aug 15 '19 at 08:48

1

var http = require('http');

http.createServer(handler).listen(8080);

var handler = function (req, res) {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello World!');
}

In this case the declared function does not yet exist.

edited Aug 15 '19 at 08:48

Suvitruf - Andrei Apanasik

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answered Oct 17 '17 at 12:29

Alisson

11
1

nodejs function hoisting : why it doesn't work?

2 Answers2