Meaning of evt = (evt) ? evt : window.event

Question

What does this JavaScript snippet mean? The (evt) part is so confusing; evt is not a boolean. How it works?

function checkIt(evt) {
        evt = (evt) ? evt : window.event
        var charCode = (evt.which) ? evt.which : evt.keyCode
        
    }

Answer 1

evt = (evt) ? evt : window.event is just the inline if syntax. It's equivalent to this code:

if (evt) {
    evt = evt;
} else {
    evt = window.event;
}

If evt is truthy, evt will be left alone. If evt isn't truthy, it will be replaced with window.event.