JavaScript uninitialised variable behaviour -

I'm learning JavaScript I had to face this behavior using the initial and unreliable variables. For example,

  // is not an initial sum initialized; For (var i = 0; i <10; i ++) {sum = sum + i; } Alert (amount); Output: 90  

< Div class = "snippet-code">

  // initial var sum = 0; For (var i = 0; i <10; i ++) {sum = sum + i; } Alert (amount); Output: 45  

Can anyone explain to me what is happening here? I think 45 times in the initial variable amount, it seems like this.

Edit: Try to run the code separately.