js left,right,mid函数

functionleft(mainStr,lngLen){
if(lngLen>0){returnmainStr.substring(0,lngLen)}
else{returnnull}
}

functionright(mainStr,lngLen){
//alert(mainStr.length)
if(mainStr.length-lngLen>=0&&mainStr.length>=0&&mainStr.length-lngLen<=mainStr.length){
returnmainStr.substring(mainStr.length-lngLen,mainStr.length)}
else{returnnull}
}
functionmid(mainStr,starnum,endnum){
if(mainStr.length>=0){
returnmainStr.substr(starnum,endnum)
}else{returnnull}
//mainStr.length
}