How to help your users out as a library designer

Seems my functional posts have been generating an obscene amount of traffic lately (thank you, Reg). In which spirit, here’s a little suggestion for anyone designing a JavaScript library.

I was reading about Oliver Steele’s MIT-licensed Functional library earlier, and came across this little gem (modified slightly to my whims):

 * Returns a function identical to this function except that
 * it prints its arguments on entry and its return value on exit.
 * This is useful for debugging function-level programs.
Function.prototype.traced = function(name, logger) {
    var self = this, name = name || this, logger = logger || 'info';
    return function() {
        window.console && console[logger](name,
                'called on', this, 'with', arguments);
        var result = self.apply(this, arguments);
        window.console && console[logger](name, '->', result);
        return result;

To which I say, never mind function-level programs (though there’s a voice in my ear whispering something about learning Haskell). It’s useful for debugging just about anything. There’s nothing stopping you using this with procedural code or class-based OO programs.

var jibber = {
    age: 68,
    getAge: function(n) {
        return "It's, er... " + (this.age/n) + ". I think.";


// console prints...
// getAge() called on Object age=68 with [2]
// getAge() -> It's, er... 34. I think.

So we see that we can use it to log method calls, not just bare functions. And it’s not even just useful for debugging. As a library author, you could use it to log interesting points in your codebase, or to highlight upcoming changes to an API in a future release:

MyClass.prototype.jibber = function() {
    return this.wobble.apply(this, arguments);
}.traced('DEPRECATED: use wobble() instead. jibber()', 'warn');

So, before releasing version 2.0 of your library, you could put out a 1.5.x release that just added a bunch of deprecation messages to make it easier for your users to upgrade from 1.5 to 2.0.

If you’ve enjoyed this article, you might enjoy my recently published book JavaScript Testing Recipes. It’s full of simple techniques for writing modular, maintainable JavaScript apps in the browser and on the server.