Javascript callbacks ordering and node

Question

Per http://callbackhell.com/ I try to write callbacks (mostly for learning purposes before I proceed to promises) The Right Way(tm).

OK so I have this:

"use strict";

var fs = require('fs');

var readFileDone = function(err, data) {
    if(err) return console.error(err);
    console.log('read done');
    writeFile(null, data);
};


var writeFile = function(err, data) {
    if(err) return console.error(err);
    var ws = fs.createWriteStream('/tmp/copy.txt')
    ws.end(data, 'utf-8', writeFileDone);
};

var writeFileDone = function(err, data) {
    console.log('writing file done');
};

fs.readFile('test_underscore1.js', readFileDone);

I run this using ST3's build system I defined for Javascript like so:

{
  "cmd": ["/usr/bin/nodejs", "$file"],
  "selector": "source.js"
}

Now the problem: if I move line fs.readFile('test_underscore1.js', readFileDone); above callback definitions, it does not run (no output in console and the file in /tmp does not get created either).

Why is that? After all, Javascript is supposed to parse entire file including function definitions before running? Therefore, the position of the line calling fs.readFile should not matter?

Answer 1

Seems to be a hoisting issue, your functions expressions are declared but not set until later on on the lines where you assigned them.

Effectively you have end up with:

"use strict";

var fs = require('fs');
var readFileDone,
      writeFile,
      writeFileDone;

fs.readFile('test_underscore1.js', readFileDone)

//  now your functions are assigned and available; after the above call

 readFileDone = function(err, data) {
    if(err) return console.error(err);
    console.log('read done');
    writeFile(null, data);
};

 writeFile = function(err, data) {
    if(err) return console.error(err);
    var ws = fs.createWriteStream('/tmp/copy.txt')
    ws.end(data, 'utf-8', writeFileDone);
};

 writeFileDone = function(err, data) {
    console.log('writing file done');
};

When you move the fs call above them you position it before they have been assigned the function definition (i.e. they aren't ready)

Answer 2

This doesn't work because javascript is ran as it is loaded. The file does not get loaded then parsed then ran. It gets ran as it is parsed.

So, you are running your fs.readFile before the callbacks are loaded into memory. This is the same with any scripting language. Compiled languages on the other hand are loaded before they are ran.

Node uses a headless browser (Chrome) to execute your scripts, so think of it as if it was running in the browser without a UI.