jsleft,right,mid函数_javascript技巧

function left(mainStr,lngLen) {
if (lngLen>0) {return mainStr.substring(0,lngLen)}
else{return null}
}

function right(mainStr,lngLen) {
// alert(mainStr.length)
if (mainStr.length-lngLen>=0 && mainStr.length>=0 && mainStr.length-lngLen<=mainStr.length) {
return mainStr.substring(mainStr.length-lngLen,mainStr.length)}
else{return null}
}
function mid(mainStr,starnum,endnum){
if (mainStr.length>=0){
return mainStr.substr(starnum,endnum)
}else{return null}
//mainStr.length
}