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The same concept works in Javascript which is described in more detail here.

This is a nice pattern in Lua to make it possible to add parameters to a function without needing to break backwards compatibility with older code.

Say we make a first version of a function and it takes two parameters one parameter like this:

function FOO(T)
   local Data = T.data
end

...

Then we realize that we need a timeout, we can extend the function to support this a second parameter without needing to alter previous code calling this function. In fact we can code things in a way that adds a default.

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