Don't understand why this code is working

Question

If the user's browser is IE, and the localStorage doesn't already exist, the following code sets a localStorage, which has an expiry date of 24 hours.

(function ieAlert() {
  var lastclear = window.localStorage.getItem('myLocalStorage'),
  time_now  = (new Date()).getTime();

  var isIE = document.documentMode

  if (isIE && !lastclear) {    
    if ((time_now - lastclear) > 1000 * 60 * 60 * 24) {
      window.localStorage.clear()
      window.localStorage.setItem('myLocalStorage', time_now)
    }
  }
})()

It works. But what I don't understand is this part:

if (isIE && !lastclear) {    
    if ((time_now - lastclear) > 1000 * 60 * 60 * 24) {
      window.localStorage.clear()
      window.localStorage.setItem('myLocalStorage', time_now)
    }
  }

Here the lastclear is undefined, how does the calculation works then?

Answer 1

Here the lastclear is undefined, how does the calculation works then?

No, it's null. getItem returns null for entries that don't exist. In a numeric context, null coerces to 0, so number - null is number - 0 is number.

(Whereas if the original author had accessed it the other way, localStorage.myLocalStorage, the value would indeed have been undefined, and the > wouldn't work because number - undefined is NaN, and all comparisons with NaN result in false.)

If I were writing the code, I wouldn't rely on that null coercion part of it, not least because it trips up future readers of the code (as it tripped you up). But that's why it works.