You’re confusing the concepts of function references and function calls, and you are not making a closure over the item ID.
If you declare this:
function onItemClick(id) { alert(id); }
This prints the reference to the function:
console.log( onItemClick );
And this prints the return value of calling that function (as it returns nothing, this equals undefined):
console.log( onItemClick(5) );
So when you’re doing this:
L.marker(....).on('click', onItemClick(id) );
the function gets called, and on() receives the return value of that function, i.e.:
L.marker(....).on('click', undefined );
What you want to do is have a function that returns a function:
function onItemClick(id) { return function(){ alert(id); } }
This way, when you do
L.marker(....).on('click', onItemClick(5) );
That will make a function call, and use the return value, which now looks like this:
L.marker(....).on('click', function() { alert(5); } );