[javascript]String添加trim和reverse方法

function trim() {
var start, end;
start = 0;
end = this.length - 1;

while(start <= end && this.charAt(start) == " " ) {
start ++;
}

while(start <= end && this.charAt(end) == " " ) {
end --;
}

return this.substring(start, end+1);
}

function reverse() {
var str = "";
var end = this.length - 1;
for(;end >= 0; end --) {
str = str + this.charAt(end);
}

return str;

}

String.prototype.trim = trim;
String.prototype.reverse = reverse;