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Editorial Team
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Asked: 2026-06-12T13:37:25+00:00

I set the prototype of Array as an instance of my , I think

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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>
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1 Answer

  1. Editorial Team

    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. 🙂

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