So it appears that Maxthon 5 throws a Syntax error for this JavaScript: function (a,) {} I have never looked at Maxthon because I thought they reused the engines from Chrome & Edge. But these two do not error on this JS which suggests Maxthon has a different engine...??
(The syntax error is caused by the "," following the argument name. You may want to test `x=function(a,){}` or `function x(a,){}` in a browser so as not to throw a different Syntax error because a function statement requires a function name.)