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From: Dolaameng on 13 Jan 2010 04:30
I am reading the book <Professional javascript for web developers> by
Nicholas C. Zakas. On page 162 he gives an example of how overwriting
the prototype with an object literal can cause the constructor testing
to fail. Here is the code in the book,
///////////////////////////////////////////////////
function Person(){
}
Person.prototype = {
name : "Nicholas",
age : 29,
job : "Software Engineer",
sayName : function () {
alert(this.name);
}
};
var person = new Person();
alert(person.constructor == Person); //false
alert(person instanceof Person); //true
///////////////////////////////////////////////////
As the author explained, the prototype of the constructor Person has
been overwritten by an object literal. As a result, person.constructor
will not evaluate to Person anymore ( it is Object now) and somehow
the instanceof operator still works. Everything seems straightforward.

But something interesting happened when I tried the code myself and
changed the execution order of the two code snippets by accident-- I
put the line of creating a new instance "var person = new Person();"
BEFORE the overwriting of prototype, so it is now,
///////////////////////////////////////////////////
function Person(){
}
var person = new Person(); //now before the prototype overwriting
Person.prototype = {
name : "Nicholas",
age : 29,
job : "Software Engineer",
sayName : function () {
alert(this.name);
}
};
alert(person.constructor == Person); //true
alert(person instanceof Person); //false
///////////////////////////////////////////////////
As a consequence, the testing for person.constructor evaluates to true
while the instanceof operator evaluates to false now.

So I guess this is what is going on behind the scenes--everytime when
a new instance is created by the constructor, the instance's hidden
__proto__ property will be dynamically set to the current prototype
property of the constructor. That is why in the second code the
instance person's constructor still points to Person.

And the 'instanceof' works somehow like this: it compares the
instance's hidden __proto__ against the prototype property of the
constructor. If the constructor's prototype property is on the
instance's __proto__ chain, it will evaluates to true. So as in the
first code, even when the prototype's constructor property is not
correctly set, the instanceof operator will still work.

I am still not quite sure about these prototype and instanceof
concepts. So any supplementary comments will be welcomed.
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