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Asked: 2026-06-17T09:04:57+00:00

I am working on a GWT project where the host page has scripts that

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I am working on a GWT project where the host page has scripts that add properties to String.prototype. The host page scripts rely on the existence of these properties in order to run correctly, so I need to make sure that whenever I pass a string from GWT to “host code”, the extra properties are present in the object.

I thought that by calling $wnd.String in a non-constructor context, the extra properties would be there, but this appears to not be the case. In this mock-GWT setup, for example:

test.html:

<!DOCTYPE html>
<html>
<head>
<script>

String.prototype.myStartsWith = function String_myStartsWith(prefix) {
    prefix = String(prefix);
    return (this.indexOf(prefix) === 0);
};

function onLoad() {
    var iframe = document.createElement("IFRAME");
    iframe.src = "javascript:''";
    iframe.id = "myFrame";
    document.body.appendChild(iframe);
    iframe.contentWindow.location.replace("myFrame.html");
}

</script>
</head>
<body onload="onLoad()">
</body>
</html>

myFrame.html:

<!DOCTYPE html>
<html><head><script>

var $wnd = parent;
var $doc = $wnd.document;

debugger;

</script></head></html>

When execution hits the debugger line, the following results are obtained using the JS debugger console:

> $wnd.String !== String
true

> new $wnd.String("test").myStartsWith === undefined
false

> $wnd.String.prototype.myStartsWith === undefined
false

> $wnd.String("test").myStartsWith === undefined
true

The results are the same in Firefox 18.0, Safari 6.0.2, and Opera 12.12.

The first three are expected, but I did not expect the fourth check, $wnd.String("test").myStartsWith === undefined, to be true.

Why is it that myStartsWith is not available in string objects returned by calling $wnd.String in a non-constructor context?

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1 Answer

  1. Editorial Team

    Why is it that myStartsWith is not available in string objects returned by calling $wnd.String in a non-constructor context?

    Because the non-constructor String function just applies ToString and returns a primitive string value. If you access a property of that, the String constructor of the current global context is used – the one without a modified prototype. So

    $wnd.String("test").myStartsWith

    is evaluated to

    "test".myStartsWith

    and is equivalent to

    new String($wnd.String("test")).myStartsWith

    which is undefined of course.

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