I don't quite get this, maybe someone can help. There's already a proposal for adding the existential operator in ECMAScript, if you're going to require babel transpilation why not simply write a babel plugin for the existential operator and use that?

If the argument is "because the proposal is immature and could be withdrawn" then the counter argument is that you have basically extended the language already by providing a built in "idx" global much like, say, parseFloat, with the difference that your function does not have any chance of actually being in the spec at any point, unlike the existential operator, thus you're basically masquerading the fact that using idx and the current proposal is the exact same thing just with different syntax.