author: Krasimir Tsonev

Hi there, I'm . Senior front-end engineer with over 13 years of experience. I write, speak and occasionally code stuff.

A bad side effect of the aspect oriented programming

I just read this very interesting article published on know.cujojs.com. It's about aspect oriented programming. At the beginning I thought "Oh, that's kinda cool", but when I start thinking about how I'll use it in practice I notice something bad. The blog post itself is well written and informative, but doesn't include any cons of the used technique.

The example, shown in the article, looks like that:

function Thing() {}
Thing.prototype.doSomething = function(x, y) {
    var result;
    // compute some result using x and y
    return result;
};

// AOP
var origDoSomething = Thing.prototype.doSomething;
Thing.prototype.doSomething = function() {
    var start = Date.now();
    var result = origDoSomething.apply(this, arguments);
    console.log((Date.now() - start) + 'ms', x, y, result);
    return result;
}

var thing = new Thing();
// some time later, and possibly even in
// another part of the application
var result = thing.doSomething(x, y);

It is actually a smart way to add behaviour to your class without touching the implementation of it. You do this without changing even a line of code.

Some of the benefits listed:

  • The source code of Thing hasn't been modified.
  • The consumers of Thing do not need to change.
  • The behavior of the original doSomething, i.e. its contract has been preserved.
  • Thing has no knowledge of doSomethingElseFirst, and no dependency on it. Thus, Thing's unit tests do not need to be updated.

However, if you imagine the real useage you will probably spot the problem. There is a modification of your function which is kinda hidden. If the new version of your method is not placed exactly below the original one, it will be difficult to find out that it exists. If there is any problem with the new behaviour you will probably start looking at the class and its methods. You will put some console.logs, but everything will look ok, because real problem is outside the class. If you use aspect oriented programming I'll suggest to put a comment inside the original function explaining that this method is changed from the outside. Something like that:

function Thing() {}
Thing.prototype.doSomething = function(x, y) {
    // ---> (AOP): modified from /lib/aop/profiler.js:243 
    var result;
    // compute some result using x and y
    return result;
};
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