I'm not a JS engine dev, but my theory is also that one of the reasons was probably the negative impact on the performance since you add 2 new fundamental types to the language which already has 9. JS is a dynamic language and this is a toll for the optimizer and runtime performance. JS has to handle and check/validate all the possible combinations of operations between different types of variables during every step. Adding 2 new fundamental types (not just some new API) complicates things significantly because you increase the complexity of the type handler logic which might also involve some unavoidable additional runtime checks.