Tuesday, August 13, 2013

Install a specific version of Node.JS

Quick post (personal reminder above all) about how to install a specific version of Node.js. I'm working on Ubuntu 12.04. The idea is to clone the whole repository and then checkout only the interested version. Then install it. Here are the commands.

git clone https://github.com/joyent/node.git
cd node
git checkout "v0.8.18"
export JOBS=2
make install

And you are good to go!

Monday, August 12, 2013

I'm still alive - !false and !undefined in JavaScript

Hi all, I've been very busy lately, and then I went for a holiday, so I didn't update the blog. Sorry!
I recently came back from the seaside and started to work on a bug I had since forever. I thought it was a deeply rooted bug that was spread in different methods. It turns out it was not like that. And that's what I discovered:

First of all, I had this array that kept count of how many distributed processes answered a pull request. Each process has an ID, and it corresponded with the index of the array. The array first is filled with false values. As an example, imagine process ID 0 that is polled. At first the value in the array at index 0 is false, but when the process answers, I set the value in the array at index 0 to true.

If the process don't reply after a certain threshold time, I execute something. I used to check this by going through the array in this way:

for(var i = 0; i < processes.length; i++)
        //do something

In other words, if array[i] is false, it means the process did not answer.
This could look correct if only I would take into account the fact that, concurrently, some processes may be spawned, thus increasing the processes array. Since I didn't polled the newborn processes, I don't want them to be checked. Of course, with the shown code, this was happening. Luckily, JavaScript fills with undefined the indexes of an array which have not be initialised; but on the other hand the evaluation of !undefined is the same as the evaluation of !false. This clearly lead to a bug which always executed something, even if it was not the case. Again, luckily with JavaScript I could correct this very easily:

for(var i = 0; i < processes.length; i++)
    if(array[i] === false)
        //do something

And that's it!