Thanks for the question

MYAPP is a global variable, when you modify
MYAPP.myevent.addListener
even from inside a function, you’re still modifying the property of a global object.

same if you go something like:
GLOBAL = 1;
function a() {
GLOBAL = 2;
}
a();
alert(GLOBAL); // 2

In the example you quote, the branching happens only once and during that first call the function overwrites itself. So the second time you use it, it has a new body which doesn’t have branching code in it.

You can easily test though. Just add a console.log call, like:

...
MYAPP.myevent = {
  addListener: function(el, type, fn){
    console.log('called');
    if (typeof el.addEventListener === 'function') {
      //....

then use the function like
MYAPP.myevent.addListener(window, 'click', function(){alert('click')});
(this will pring “called” in the console)
and then again one more time
MYAPP.myevent.addListener(window, 'click', function(){alert('click 2')});
(this will not print in the console)

Hope that helps!

S

At page 273 ,chapter 8 .

Your code like this :

*****************************
var MYAPP = {};
MYAPP.myevent = {
addListener: function(el, type, fn){
if (typeof el.addEventListener === ‘function’) {
MYAPP.myevent.addListener = function(el, type, fn) {
el.addEventListener(type, fn, false);
};
} else if (typeof el.attachEvent === ‘function’){
MYAPP.myevent.addListener = function(el, type, fn) {
el.attachEvent(‘on’ + type, fn);
};
} else {
MYAPP.myevent.addListener = function(el, type, fn) {
el['on' + type] = fn;
};
}
MYAPP.myevent.addListener(el, type, fn);
}
};

************************************
This code should be runnable , but does addListener() only be defined once like this ?

There is no return statement in your code , how can the subsequent calls of addListener() avoid running the branching?

Notify through email please . Thanks

h instanceof Object is indeed true

Also

>>> Object.prototype.isPrototypeOf(Hero)
true

When I was reading the book “Object-Oriented Javascript”, at page 103 ,Chapter 4 , I doubt the correctness of your examples.

You said :

>>> function Hero(){}
>>> var h = new Hero();
>>> var o = {};
>>> h instanceof Hero;
true
>>> h instanceof Object;
false
>>> o instanceof Object;
true

How can “h instanceof Object” be false ? I guess you want to say o instanceof Hero be false , am I right ?

Notify me through email ,please .

data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAQAAAC1+jfqAAAA20lEQVR4XmXN3ypFQRSA8cVzUN5hnZ7KioP4EYipdIJnahDhAokuSU6J5v9d8xsn9VUk5m9vsv1mzWisySZ5Ekrsihhsq6wvSNEx9rqf5LbvqL2ffHJO4431gNZkNzRYmi1koIPHA0VG0oCMEwYs88uW2yGryTXrQdYnGbpDczPniNgeOME445YJ89dthmngItXGko+U7BNRdMOWXCEYeMaWIQ3zBal4IHbrnhikvOtekA+Kz2E0rAC8888cg9dz4bgV8lw3oFfkamdv1wbUrJxM+yjNQOy3QjfgORR5brtfZAAAAAElFTkSuQmCC

Good point about optimizing the png