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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>
817
You 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.net/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. :)
159
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