Evaluation of if statement returns different values

To reproduce, open the attached file in qmlscene.

Given that var key is a number returned from a function, and
var keycheck = (key !== void 0 && key !== -1)

These two if statements produces different outputs
if (key !== void 0 && key !== -1) and if (keycheck)

This is reproducible on 32 bit windows, but not on 64 bit mac.