I set the prototype of Array as an instance of my, I think book.aa will display "aa", but it displays "undefined", why? Thanks!
<html>
<head>
<title>Array Properties</title>
<h2>Array Properties</h2>
<script type="text/javascript">
function my() {
this.aa = 'aa';
}
Array.prototype = new my();
Array.prototype.bb = "bb";
var book = new Array();
book[0] = "War and Peace";
</script>
</head>
<body bgcolor="lightblue">
<script type="text/javascript">
document.write(book.aa+book.bb);
</script>
</body>
</html>
I set the prototype of Array as an instance of my, I think book.aa will display "aa", but it displays "undefined", why? Thanks!
<html>
<head>
<title>Array Properties</title>
<h2>Array Properties</h2>
<script type="text/javascript">
function my() {
this.aa = 'aa';
}
Array.prototype = new my();
Array.prototype.bb = "bb";
var book = new Array();
book[0] = "War and Peace";
</script>
</head>
<body bgcolor="lightblue">
<script type="text/javascript">
document.write(book.aa+book.bb);
</script>
</body>
</html>
2 Answers
Reset to default 7You cannot assign to Array.prototype because prototype is a read-only property of Array.
So when you write
Array.prototype = new my();
nothing happens. To see why, try
JSON.stringify(Object.getOwnPropertyDescriptor(Array, "prototype"))
The result is
"{"value":[],"writable":false,"enumerable":false,"configurable":false}"
Unless you are in strict mode, the assignment silently fails.
That's why -- and see http://jsfiddle/5Ysub/ -- if you execute
function my() {
this.aa = 'aa';
}
Array.prototype = new my();
Array.prototype.bb = "bb";
var book = new Array();
book[0] = "War and Peace";
document.write(book.aa+book.bb);
You get
undefinedbb
The bb works because you have assigned to the real Array.prototype when you created and set the bb property.
It is a good thing that Array.prototype cannot be whacked, IMHO. :)
Even if you could override Array.prototype directly like that, you would lose access to all the built-in methods such as splice, slice, push, shift, pop, unshift, sort, reverse and many others... So it would be terrible coding practice. But it doesn't work as Ray pointed out because it is read only.
This tiny code piece will demonstrate that Array.prototype cannot be overridden because the sort method will still execute:
<script type="text/javascript">
Array.prototype={};
var a=new Array(1,4,5,7,8);
a.sort();
alert(a.join(","));
</script>
If you want to override prototype properties for Array.prototype, you must do it one at a time like so:
Array.prototype.aa='aa';
If you want to apply a large amount of properties to Array.prototype, then apply it through a loop. Here is some code I wrote for you which should acplish exactly what you are trying to do:
<script type="text/javascript">
function my()
{
this.aa = 'aa';
}
my.prototype.bb = "bb";
var instance = new my();
for(var j in instance)
{
if(instance.hasOwnProperty(j) || j in my.prototype)
{
Array.prototype[j]=instance[j];
}
}
var book=new Array();
book[0]="War and Peace";
alert(book.aa);//alerts "aa"
alert(book.bb);//alerts "bb"
</script>