Question

I have this code:

function getSessionGUID() {
   return (S4()+S4());
}

function S4() {
   return (((1+Math.random())*0x10000)|0).toString(16).substring(1);
}

It clearly returns a string, but if you run it a bunch of times, you can notice that it sometimes returns infinity.

for (var i = 0; i < 100000; i++){ if(getSessionGUID() == Infinity) console.log("INFINITY"); }
871 x INFINITY

I then noticed that if you remove the |0, it solves the problem:

function S4() {
   return (((1+Math.random())*0x10000)|0).toString(16).substring(1);
}

Results:

for (var i = 0; i < 100000; i++){ if(getSessionGUID() == Infinity) console.log("INFINITY"); }
undefined

Why does this happen? In both cases the value is changed into a string.

Answer 1

This test reveals the answer:

for (var i = 0; i < 100000; i++){
  var x=getSessionGUID();
  if(x == Infinity) console.log(x); }
}

It logs values like 61e93284 or 1413e390

These values are valid numbers and they are way too big for the Number type, so they get cast as Infinity when interpreted as a number.

If you replace == with === in the test, no conversion occurs and nothing is logged. The conversion is caused by the == operator.