JavaScript related operation instructions on select_form effects

1. Insert option
1. DOM method
var oSelectYear = document.getElementById("SelectYear");
var op = document.createElement("option");
op.innerHTML = "2010";
op.value = "2010";
oSelectYear.appendChild(op);

2. new Option method
var oSelectMonth = document.getElementById( "SelectMonth");
oSelectMonth.options.add(new Option(1, 1));
oSelectMonth.options.add(new Option(2, 2));

2. Clear options
var oSelectMonth = document.getElementById("SelectMonth");
oSelectMonth.options.length = 0; //Clear options in Select

3 , Set the default selected option
var oSelectMonth = document.getElementById("SelectMonth");
//oSelectMonth.selectedIndex = 1; //Method 1: The second item is selected by default
//setTimeout (function() { oSelectMonth.selectedIndex = 1; }, 0); //Use setTimeout delay to prevent dom rendering problems
// oSelectMonth.options[1].selected = true; //Method 2
oSelectMonth.options[1].setAttribute("selected", "true"); //Method 3: It is recommended to use setAttribute to set