Using a C++ class inside Javascript without an .idl

It is possible to call methods on a class whose interface is not exposed through an .idl file?

Im my particular case, I'd like to use the @mozilla.org/gfx/fontmetrics;1 class, whose interface nsIFontMetrics doesn't have a correspondent .开发者_如何学Pythonidl file.

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Nope. If there is no idl file, it isn't exposed to script through XPConnect.