JavaScript 实现自定义 trim 方法：去除字符串前后空格

//clear the right and left space
function trim(s){ 
 return trimRight(trimLeft(s)); 
} 
 
//clear the left space
function trimLeft(s){ 
 if(s == null) { 
  return ""; 
 } 
 var whitespace = new String(" \t\n\r"); 
 var str = new String(s); 
 if (whitespace.indexOf(str.charAt(0)) != -1) { 
  var j=0, i = str.length; 
  while (j < i && whitespace.indexOf(str.charAt(j)) != -1){ 
   j++; 
  } 
  str = str.substring(j, i); 
 } 
 return str; 
} 
 
//clear the right space 
function trimRight(s){ 
 if(s == null) return ""; 
 var whitespace = new String(" \t\n\r"); 
 var str = new String(s); 
 if (whitespace.indexOf(str.charAt(str.length-1)) != -1){ 
  var i = str.length - 1; 
  while (i >= 0 && whitespace.indexOf(str.charAt(i)) != -1){ 
   i--; 
  } 
  str = str.substring(0, i+1); 
 } 
 return str; 
}