取子串及去除首尾空格

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

function ltrim(s){
return s.replace( /^\s*/, "");
}
//去右空格;
function rtrim(s){
return s.replace( /\s*$/, "");
}
//去左右空格;
function trim(s){
return rtrim(ltrim(s));
}