Unless I am mistaken, FireFox allows optional parameters in functions, Chrome does not.

gistfile1.js

<script type="text/javascript">
	function optional(required, optional = 5)
	{
		console.log(required, optional);
	}
	
	optional("a", "b");
	optional("c");
	optional();
</script>

/* ----------------------------------

All sample outputs are on Ubuntu 13.04 (though as all browsers are cross-platform, it shouldn't matter anyway)

> Mozilla FireFox 21.0
a b
c 5
undefined 5

> Google Chrome 27.0.1453.93
Uncaught SyntaxError: Unexpected token = 

*/