js 字符串截取后几位

var str = 'abcdefgh';
var arr = str.split('');
arr = arr.reverse();
str = arr.join('');
var result = str.substring(0,4);

var x = '121221212131312312ewsqdasdsa';
x.substring(x.length-4)  //sdsa

var x = '121221212131312312ewsqdasdsa';
x.slice(-4)  //sdsa