javascript - What is the parameter to use for event? how do i know?

Question

element.onkeypress = function(e) {
                    if(e.keyCode) {
                        element.keyCode = e.keyCode;
                    } else {
                        element.keyCode = e.charCode;
                    }
                };

Also in java script , there is also

<input onChange="a(event)"/>
<script>
function a(event) { 
    alert(event.target.value);
}
</script>

As a parameter receiving, how do I know if I must put event for parameter instead of e? Second example wont work if it's the parameter is anything other than event aren't both javascript?

Answer 1

When you bind an event handler using an on* property or addEventListener, the event object will be passed as the first argument. You name it yourself as is usual when writing a function expression or function declaration. The normal restrictions on what you can name arguments apply (i.e. they must be valid identifier names). event, e and evt are common names for that variable.

When you bind an event handler using an on* attribute, you are writing only the function body (i.e. function (event) { and } are implied. The event object will be available in the event variable.