js怎么给下拉框默认选中

 <select id="shipping" name="node_type" class="valid">
                                   <option value="1">1</option>
                                   <option value="2">2</option>
                                   <option value="3">3</option>
                                   <option value="4">4</option>
                                   <option value="5">5</option>
                                   <option value="6">6</option>
                            </select>

<script>
 function selectNumber(){
                        var num =1; //获取关联id值
                        var numbers = $("#shipping").find("option"); //获取select下拉框的所有值
                        for (var j = 1; j < numbers.length; j++) {
                            if ($(numbers[j]).val() == num) {//用select所有值进行匹配跟num值进行匹配,默认选中给selected属性
                             $(numbers[j]).attr("selected", "selected");
                            }
                        } 
                    }
selectNumber();
</script>