One problem with JavaScript development is that the string representation of a value doesn’t tell you much about the value. For example, [null], [undefined], and '' all display as the empty string. [1,2}, [[1,2]], and [[^1],[^2]] all display as 1,2 (and so does "1,2"). And ({a: 1}), ({b: 2}), and new MySwankyNewObject() all display as [object Object].

If you use an IDE for development, this may not be a problem. Probably the IDE has its own string representation; even if it doesn’t, you can generally drill into objects by clicking on them. This doesn’t help those us of who prefer REPL development or printf-style debugging. When you display a debugging value (to the browser status line, to the alert() dialog, or to the Rhino console), you’d like some indication of what it actually is. And JavaScript doesn’t generally tell you, at least when the value is more complex than a string, number, or boolean.

To make it easier to use the Readable class, Readable comes with a couple of hooks. First of all, it defines defines info, warn, error, and debug functions1 that display their arguments to the user. In Rhino, these functions call through to print. In a browser, they use alert() — unless fvlogger has been loaded first, in which case they use it instead[]. You can also replace Readable.log(level, message) or Readable.display(message) to add your own behavior; for example, to display the message in the status line, or AJAX it up to the server.
Secondly, Readable can add toString methods to Array.prototype and Object.prototype. Do this, and evaluating an expression in Rhino writes a readable representation to the console, without your having to wrap it in info(...) or Readable.toString(...). Doing this has the consequence that iterating over the properties of an Object or Array will yield an extra one (toString), so this is off by default. But define READABLE_REPLACE_TOSTRING before loading the file, or invoke Readable.replaceToString() after loading it, and you’ll get this behavior.