Conditionally loading modules is also built into YUI. For example, when you
want to use JSON, YUI checks if the native version is present and if not it
loads a JS implementation.

You have to do it feature by feature. I recently wrote a Taskjs like
generators+promises implementation and it checks which syntax is supported
(ES6 or the old Firefox one without function*) by using eval() and catching
syntax errors. See
https://github.com/juandopazo/yui3-task/blob/master/js/task.js#L43-L55. It
doesn't load a new module based on that check because the code is not big
enough to warrant loading a whole new script.

Juan
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Conditionally loading modules is also built into YUI. For example, when you
want to use JSON, YUI checks if the native version is present and if not it
loads a JS implementation.

You have to do it feature by feature. I recently wrote a Taskjs like
generators+promises implementation and it checks which syntax is supported
(ES6 or the old Firefox one without function*) by using eval() and catching
syntax errors. See
https://github.com/juandopazo/yui3-task/blob/master/js/task.js#L43-L55. It
doesn't load a new module based on that check because the code is not big
enough to warrant loading a whole new script.