Sidestepping JavascriptCore's JSEvaluateScript

JSEvaluateScript takes a parameter to use as the this object, but it always uses the global object instead. Oups ! The bug is known and marked as fixed but unfortunately my version of JavascriptCore still fails.

Why use JSEvaluateScript anyway ? A quick example : to know if your object (a JSObjectRef) is an array, evaluate this.constructor == Array.prototype.constructor with this as your object and voilà ! JSValueToBoolean will then give you the answer. If you couldn't do that, you'd have to get your object's constructor property, then get a reference to the global Array class, then get its prototype property, then … well you get the idea. This kind of defeats the simpleness of Javascript :)

To work around that, create a quick anonymous function and call it. Instead of using this to get to your object, pass it as an argument. Then return your result.

	JSStringRef scriptJS = JSStringCreateWithUTF8CString("return arguments[0].constructor == Array.prototype.constructor");
	JSObjectRef fn = JSObjectMakeFunction(ctx, NULL, 0, NULL, scriptJS, NULL, 1, NULL);
	JSValueRef result = JSObjectCallAsFunction(ctx, fn, NULL, 1, (JSValueRef*)&jsObject, NULL);

It's anonymous, it specifies zero parameter names (therefore use arguments[0] to get your object), and casts jsObject to a C array of length 1 with (JSValueRef*)&jsObject.

One benefit of using a function is using multiple arguments, eg a string and a pattern to match. Although at that point it may be better to have your functions in an external js file and call these.

And more good news : JSCocoa now bridges js arrays and js hashes to NSArray and NSDictionary. Therefore Core Animation code like layer.filters = [filter] should work !

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Planet Cocoa

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