My New Best Friend •

My New Best Friend •

My New Best Friend •

A friend of mine recently pointed me at a well hidden command line tool. In the JavaScript framework used by Safari and other parts of Apple’s products, there is a tool called jsc. It’s a command line interface for JavaScript that uses the same code as the rest of the system.

You can find the binary at /System/Library/Frameworks/JavaScriptCore.framework/Versions/Current/Helpers/jsc. That path is unwieldy, so I have an alias set up that lets my just type jsc in the Terminal.

So what can you do with jsc? Pretty much anything you can do with JavaScript in a browser with the caveat that there aren’t document and window instances.

If you run jsc -h, you’ll see a lot of options for testing and profiling JavaScript. It’s clear that the WebKit team uses this internally for running tests. But we can also use it for trying out ideas and running simple utilities.

A picture is worth a thousand words, so let me show you how it can be used to solve a simple problem. I recently needed to convert some strings in our Turkish localization of Frenzic to uppercase: the lowercase “i” was getting converted to the dotless version.

JavaScript’s toLocaleUpperCase() function is the perfect way to do this, so I pulled jsc out of my tool bag and got to work. The first challenge was getting input.

Luckily, there is a readline() function that takes keyboard input and returns a value. Unluckily, that input isn’t in the encoding you’d expect it to be: characters are returned in ISO-8859-1 (Latin-1), not UTF-8. Remember, there’s no document instance so the default encoding is used.

To workaround this limitation, you can percent escape the characters to UTF-16 and then decode them back into UTF-8 with this technique:

var text = decodeURIComponent(escape(readline()));

(If any WebKit engineers are reading this, it would be nice to have a command line option like --encoding=utf-8.)

Generating output is a bit different than a browser, too. You’ll be using print() instead of console.log(). To convert the text input and display it, I used this:


There are a few more built-in functions that may prove useful, but so far, I’ve only needed to read and write text. It’s undocumented, but jsc also takes standard input and can be used as a shebang:

$ echo "print(1+2);" | jsc

Since this is likely code I’ll have to use again, I created a Turkish.js file:

while (true) {
    print('Turkish text?');
    var text = decodeURIComponent(escape(readline()));

I can now run this any time with jsc Turkish.js. And you also get to see how having JavaScript in a command line can be handy. Enjoy!

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