form 提交之前如何验证数值文本框的内容全部为数字

                            var value = $（"id"）.value;

                            var len =value.length;

                            for(vari=0;i<len;i++)

                            {

                                          if(value.charAt(i)>"9"|| value.charAt(i)<"0")

                                          {

                                                        alert("含有非数字字符");

                                                        returnfalse;

                                          }

                            }

                            return true;