Javascript 操作select下拉框

// 1.判断select选项中 是否存在Value="paraValue"的Item        
function jsSelectIsExitItem(objSelect, objItemValue) {        
    var isExit = false;        
    for (var i = 0; i < objSelect.options.length; i++) {        
        if (objSelect.options[i].value == objItemValue) {        
            isExit = true;        
            break;        
        }        
    }        
    return isExit;        
}         

// 2.向select选项中 加入一个Item        
function jsAddItemToSelect(objSelect, objItemText, objItemValue) {        
    //判断是否存在        
    if (jsSelectIsExitItem(objSelect, objItemValue)) {        
        alert("该Item的Value值已经存在");        
    } else {        
        var varItem = new Option(objItemText, objItemValue);      
        objSelect.options.add(varItem);     
        alert("成功加入");     
    }        
} 

// 3.从select选项中 删除一个Item 
function jsRemoveItemFromSelect(objSelect, objItemValue) {        
    //判断是否存在        
    if (jsSelectIsExitItem(objSelect, objItemValue)) {        
        for (var i = 0; i < objSelect.options.length; i++) {        
            if (objSelect.options[i].value == objItemValue) {