Test of js engine optimization. V8 will produce machine-code of well-used functions. This unfortunately doesn't seem to run as well in Firefox

chrome

> var fTst = (v)=>((v - lomagic) & ~v & himagic)!=0;
  himagic = 0x80808080;
  lomagic = 0x01010101;
  var fls = [], dt = [], a = 0x500000;
  for(let i=0; i<20*a; i++){
    if(fTst(i))fls.push(i);
    if(i%a==0)dt.push(+new Date);
  }//00000000
  dt.map((v,i)=>v-dt[i-1])
  
> (20) [NaN, 227, 259, 166, 67, 103, 62, 57, 59, 66, 72, 60, 58, 58, 56, 54, 75, 60, 57, 59]
> (20) [NaN, 184, 267, 227, 28, 19, 20, 20, 17, 17, 19, 17, 17, 17, 15, 17, 16, 17, 16, 18]