Js function and arrow inside the map function usage

var a = [ 
'Hydrogen', 
'Helium', 
'Lithium', 
'Beryl­lium' 
];
//获取每个元素的长度，组成一个数组，方法1
var b = a.map(function(s){return s.length});
//方法2
var c = a.map(s => s.length)