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For our implementation, we have to take into account that in a browser, code loading cannot be done synchronously "on demand". Thus we use AMD and map compilation units to AMD modules. The AMD loader resolves all static dependencies and loads the code of all compilation units that are potentially needed at runtime. We have to take care of initializing those compilation units only when they are actually used. This determines the value of a compilation unit the corresponding AMD module returns: Instead of the primary declaration itself, the return value is a compilation unit object from which you can retrieve the primary declaration. The trick is that the compilation unit self-initializes when the primary declaration is first requested!.

We could implement the primary declaration request by a method of the compilation unit object. But this would involve a function call for each retrieval of the primary declaration, even if the compilation has already been initialized. A trick for an efficient implementation is to use a get property that, upon its first invocation, initializes the compilation unit and replaces itself by a simple field with a direct reference to the primary declaration. For the sake of brevity and readability (we want the reader to mainly ignore this helper property), let's call that property _ (underscore).

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This is not only a performance / memory consumption problem, as in AS3, the method keeps is its identity. This is important for the typical use case that bound methods are registered as event listener callbacks. When trying to remove an event listener, identity has to be ensured, or the function will not be found and the event listener is not removed.

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Note that in case no type check matches, the error has to be re-thrown! . This code can be omitted if the original code contains a catch clause with an untyped or * typed error variable.

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