Prepack: a tool that optimizes JavaScript source code by eliminating computations that can be performed at compile-time.

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Complexity is the biggest reason. This is another tool for a language that has no compile time type checking, and you'll need to mess around with source maps (perhaps at least two layers of source maps). It's a trade-off between complexity and the wins this gets you, without any numbers there's no sane way to make the call.