Question

The following code is from ‘JavaScript by Example Second Edition’ EXAMPLE 15.24
I don’t understand the code var evt = e || window.event; //Browser differences
I think the var evt should be true or false, so evt.target should be null, right?

function colorText(e){
  var evt = e || window.event; //Browser differences
  var evtTarget= evt.target || evt.srcElement;
  if(evtTarget.id=="first"){
div1.className="red";
}
else if(evtTarget.id == "second"){
div2.className="blue";
}
else{ div3.className="green";}
}

Answer 1

The keyword expression returns the value of the first expression operand if it’s not null falsey, otherwise the value of the second.

Javascript is weak typed, so you can always use || as a boolean, because everything which isn’t false or null evaluates to true. But it’s actually returning the value here, s. So evt becomes either e (the argument) or if e doesn’t exist has a falsey value, it becomes window.event.