With all my recent Javascript experiments, I wanted a browser-independent Javascript console more and more. I found Rhino and have been using it for the last month or so. It’s proven pretty useful, and it was quite easy to install: I simply copied js.jar to /usr/local/rhino/ and then added the following to my .profile for convenience: alias js="java -jar /usr/local/rhino/js.jar"

I just learned about jsc, a console Apple includes with the JavascriptCore library that I believe every version of OS X has installed by default. It’s found at /System/Library/Frameworks/JavaScriptCore.framework/Versions/A/Resources/jsc, so again you’re going to benefit from an alias or adding the directory to your PATH.

While Rhino has more features, namely the ability to load Java libraries, the jsc experience is better: the console strikes me as more responsive (it definitely launches much faster), a command history is available and you can use ctrl+a and ctrl+e to jump to the beginnings and ends of lines. Whoever made jsc seems to have a functional bent, as every expression’s return value is printed out. This means you get undefined on screen a lot if you’re doing lots of loops and/or prints, which can be annoying.

Update, 2010-06-22: Using rlwrap (e.g. alias js="rlwrap java -jar /usr/local/rhino/js.jar") solves many of the problems I had with Rhino.

Speaking of that, I think they handle the return values of loops incorrectly. As I understand it, all loops should return undefined, i.e. nothing. But consider this jsc session:

> var a = 1; while (a < 4) { print(a); a++; }
1
2
3
3

3 is printed out twice because it is ‘returned’ by the while loop. Of course, while does not really return a value, as this session shows:

> var a = 0; var b = while (a < 4) { a++; }
Exception: SyntaxError: Parse error

But where does the 3 come from, the a variable or the loop’s condition? From the following sessions, I believe it comes from the last time the condition evaluates to true:

> while (1 != 1) {}
undefined
> var a = 1; while (1 != 1) { a; }
undefined
var a = 1; while (1 != 1) { a = 2; }
undefined
> var a = 1; while (a != 2) { a = 2; }
2

Note that when the condition never is true, there is no defined value outputted. Likewise, assignment within the loop has no effect on what is ‘returned’. Or rather, I wish it was that way. Consider this:

> var a = 0; while (a < 2) { a = 2; }
2
> var a = 0; while (a < 2) { a++; }
1

To be honest, I really am confused about what jsc is doing here. It seems wrong, though irrelevant since these values printed out cannot be assigned to a variable and used. Perhaps someone from the jsc team can explain this?