Numeric support. Direct support for 64-bit integers would enable languages to efficiently represent a broader range of numeric types in the browser. The DOPPIOJVM uses a comprehensive software implementation of 64-bit integers to bring the long data type into the browser, but it is extremely slow when compared to normal numeric operations in JavaScript.
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how far is this from being solved?
There is a proposal for value types [0] that Niko Matsakis [1] and Brendan Eich [2] talked about a couple of years back and could be used to implement 64-bit numbers in JS, but I'm still waiting for an implementation or a more complete proposal. Looks like it might be dead in the water, as the proposal hasn't been edited in quite some time, but I do not have my finger on the pulse of browser standards so I could be wrong.
[0] http://wiki.ecmascript.org/doku.php?id=strawman:value_object...
[1] http://smallcultfollowing.com/babysteps/blog/2014/04/01/valu...