substr

var s = 'abcd'
var res = s.substr(0,s.length-1)  //从索引0开始，截取s.length-1个字符
console.log(res)

substring

var s = 'abcd'
var res = s.substring(0,s.length-1) //从索引0开始，截取至索引下标为s.length-1的字符，包左不包右
console.log(res)