js文字列の常用操作

//1          
//string         
//type            
//num        
function stringReduce(string='abcdefghijklmn',type='start',num=1) {
    if(num>string.length){
        return 0
    }else{
        if(type==='start'){
            string = string.substr(num,string.length-1)
        }else if(type==='end'){
            string = string.substr(0,string.length-num)
        }
        return string
    }
}

function reverseString(string='abcdefg'){
    return string.split('').reverse().join('')
}