js 获得字符串字符数，截取一定字符的字符串

//计算字符串长度    方法一：
	String.prototype.strLen = function() {
	    var len = 0;
	    for (var i = 0; i < this.length; i++) {
	        if (this.charCodeAt(i) > 255 || this.charCodeAt(i) < 0) len += 2; else len ++;
	    }
	    return len;
	}
	//计算字符串长度    方法二：
	 String.prototype.strLen = function() {
	     var len = 0;//字符串长度
	    for(var i =0; i < this.length; i++) {
		    var charStr = this.charAt(i);
		    if (/^[\u0000-\u00ff]$/.test(charStr)) {//一个字节
		    	len += 1;
			} else {
				len += 2;
			}
	    }
		return len;
	 }
	//将字符串拆成字符，并存到数组中
	 String.prototype.strToChars = function(){
	     var chars = new Array();
	     for (var i = 0; i < this.length; i++){
	         chars[i] = [this.substr(i, 1), this.isCHS(i)];
	     }
	     String.prototype.charsArray = chars;
	     return chars;
	 }
	 //判断某个字符是否是汉字
	 String.prototype.isCHS = function(i){
	    var charStr = this.charAt(i);
	    if (/^[\u0000-\u00ff]$/.test(charStr)){
	         return false;
	    }
	     else{
	         return true;
	     }
	 }
	 //截取字符串（从start字节到end字节）
	 String.prototype.subCHString = function(start, end){
	     var len = 0;
	     var str = "";
	     this.strToChars();
	     for (var i = 0; i < this.length; i++) {
	         if(this.charsArray[i][1])
	             len += 2;
	         else
	             len++;
	         if (end < len)
	             return str;
	         else if (start < len)
	             str += this.charsArray[i][0];
	     }
	     return str;
	 }
	 //截取字符串（从start字节截取length个字节）
	 String.prototype.subCHStr = function(start, length){
	     return this.subCHString(start, start + length);
	 }