In most browsers, `event` is an ever-existing JavaScript variable which value changes depending on context

event.md

In most browsers, event is an ever-existing JavaScript variable which value changes depending on context.

Safari, Chrome and Opera:

<script>
  window.onload = function() {
    console.log(event) // Event (type: "load")
  }
  console.log(event) // undefined
</script>

Internet Explorer (tested 7 to 9):

<script>
  window.onload = function() {
    alert(event) // Event (type: "load")
  }
  alert(event) // null
</script>

Firefox:

<script>
  window.onload = function() {
    console.log(event) // ReferenceError: event is not defined
  }
  console.log(event) // ReferenceError: event is not defined
</script>

This means, if it wasn’t for Firefox, you wouldn’t have to declare an event or e argument for event handlers. You could safely declare function() and use event inside the function body.