Simple JavaScript IDE

There are a lot of good online IDEs at the moment. One of my favorites is JSFiddle, but you said you don't want to use CSS and HTML so it's superfluous in this case. You can use JSBin, opening only the Javascript and Console panels. Another very good one is, in my opinion, Ideone, which has a lot of languages(for JS you have to select Javascript Spidermonkey). The last one I suggest you is JSConsole, from the creator of JSBin, which is basically an enhanced Javascript console(as the name states).

Ended up using this IDE as I could run it on a low powered Laptop. Free Version

http://brackets.io/

There is also a free offering from Microsoft with Visual Studio 2015 called "Visual Studio Code" runs on Macs and linux too. Only used it for a day but it work great on my low powered tablet.

https://code.visualstudio.com/

Have a look at either,

• WebStorm: real JavaScript IDE (possibly the best.)
• Sublime Text 2: text editor with syntax highlighting, and possibility to install plugins.

You can use either software, together with node.js, to get what you want.

Executing the file,

function squareNumber(x) {
    return x * x;
}

console.log(squareNumber(5));

with node.js will output,

25