I've been experimenting with JavaScript's world-renowned prototypical inheritance implementation. So far, everything makes sense to me except for one thing...
function base() {
this.a = "baseObj property";
}
base.thisIs = "base constructor";
function derived() {
this.b = "derivedObj property";
}
derived.thisIs = "derived constructor";
var baseObj = new base();
So far so good. baseObj.a returns "baseObj property" and baseObj.constructor.thisIs returns "base constructor".
But things begin to confuse me when I actually make something inherit the base object's values.
derived.prototype = baseObj;
derivedObj = new derived();
What ends up happening, is derivedObj.a returns "baseObj property". Good. derivedObj.b returns "derivedObj property". Good again. But derivedObj.constructor.thisIs returns "base constructor"...
For this to be happening, the interpreter must be failing to find derivedObj.constructor in derivedObj. So what it does, is it follows derivedObj.__proto__ to search there instead. Because the new keyword sets derivedObj.__proto__ equal to derived.prototype, which we set earlier to equal baseObj, derivedObj.__proto__ ends up returning baseObj.
This would explain why derivedObj.constructor has been forgotten about. It's seemingly useless. Interpreters don't need to use it to obtain derived.prototype; they can just use derivedObj.__proto__. The reality however, is that it could have been useful to use derivedObj.constructor in order to obtain the value of derived.thisIs.
But even after all that. It doesn't explain why it hasn't been forgotten in baseObj. Why does a .constructor exist within baseObj but not derivedObj? They were initialized the exact same way. With the new keyword.
By default, functions have a prototype property, which is the object from which the instances inherit when the function is used as a constructor. That prototype object has a constructor property, which points back to the function.
The problem with derived.prototype = baseObj is that you replace the entire prototype, so you lose the original derived.prototype.constructor, which returned derived.
A way to solve it would be reassigning that property:
derived.prototype = baseObj;
derived.prototype.constructor = derived;
However, that will alter the baseObj object. This is usually undesired, so the proper way is
derived.prototype = Object.create(baseObj);
derived.prototype.constructor = derived;
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