Why does this event handler use "e = e || event"?

Question

Can someone explain me what does this line of code means:

function(e) { e = e || event; e.returnValue = false; return false; }

Why is the parameter named e?
If I change it to 'myparam' will it work?
What does e = e mean?

Where is the variable event ( after || ) declared ? What is e.returnValue?

Answer 1

This is all basic event management, although it is missing e.preventDefault()...

To break it down, when an event handler is fired:

• Some browsers pass a parameter to the callback holding event data (this is the standards-compliant way of doing it)
• Other browsers (mostly old IE) instead put the event data in window.event (which is accessed here with just event, which is risky since that relies on there being no local variable with that name)

Next, e = e || event; is a standard way of saying "if the parameter was not passed, default it to whatever's after the ||". In this case, if the event parameter is not passed, then it looks for the global variable.

e.returnValue is one of three ways to stop an event from causing its default action. The other two are e.preventDefault && e.preventDefault() (which is conspicuously absent from the code you posted), and return false;