Javascript 如何通过选中复选框和下拉列表更改标签值

我有2个复选框，其中一个包含5个元素的下拉列表

复选框A 复选框B：下拉列表 文本框 首先要检查checbox B是否被选中，然后文本框只能显示值，否则为空。 其次，如果选中复选框B并从下拉列表中选择值，则文本框必须立即在更改中显示值

<input type="checkbox" name="events" value="event02 />
Event 01
</input>
<br/>
<input type="checkbox" name="events" value="event02 />
Event 02:
<select name="selectOne" onchange="amount()">
    <option value="00" selected="selected">00</option>
    <option value="01">01</option>
    <option value="02">02</option>
    <option value="03">03</option>
    <option value="04">04</option>
    <option value="05">05</option>
    <option value="06">06</option>
    <option value="07">07</option>
</select>
<br/>
<input type="text" name="displayTxtBox" id="displayTxtBox" size="10" maxlength="50" value="" />

在Javascript中，我不知道如何编写代码来检测复选框B detected和下拉列表selected only display value on Textbox

我认为它需要onChange和一些Javascript函数

---

请指导我。

给定更正的HTML，添加id属性和标签元素，为各种表单元素提供额外的点击目标：

<input type="checkbox" id="input1" name="events" value="event01" />
<label for="input1">Event 01</label>
<br/>
<input type="checkbox" id="input2" name="events" value="event02" />
<label for="input2">Event 02</label>

<select name="selectOne">
    <option value="00" selected="selected">00</option>
    <option value="01">01</option>
    <option value="02">02</option>
    <option value="03">03</option>
    <option value="04">04</option>
    <option value="05">05</option>
    <option value="06">06</option>
    <option value="07">07</option>
</select>
<br/>
<input type="text" name="displayTxtBox" id="displayTxtBox" size="10" maxlength="50" value="" />​

下面的JavaScript似乎对您的预期结果有效，尽管我不能完全确定您的意图：

function showAmount(el, selectEl, targetEl, clearOnUncheck) {
    /* the following is an optional, Boolean, variable, so a ternary
       conditional is used, which checks that 'clearOnUncheck' exists, 
       if it does then the variable is assigned its own value,
       if *not* then it's explicitly assigned the value of false
       note it's Boolean, *not* a string: don't quote this value */
    var clearOnUncheck = clearOnUncheck ? clearOnUncheck : false;
    if (!el || !selectEl || !targetEl) {
        // sanity checking, these are *required* arguments for the function
        return false;
    }
    // if the checkbox is checked, continue
    else if (el.checked) {
        /* assigning the conditions I'm testing to variables, so if conditions
           change they only have to be changed in one place */
        var index = selectEl.selectedIndex,
            textcontent = (typeof(el.textContent) != 'undefined');
        if (textcontent) {
            // up-to-date browsers
               /* sets the value of the text-input to be the string contained
                  within the selected option from the select element */
            targetEl.value = selectEl
                .getElementsByTagName('option')[index]
                .textContent;
        }
        else if (window.innerText) {
            // IE < 8
               // as above, but uses innerText for IE
            targetEl.value = selectEl
                .getElementsByTagName('option')[index]
                .innerText;
        }
    }
    /* if the checkbox is unchecked, and you've set the Boolean for the
       optional clearOnUncheck to true (remember, *not* a string, don't quote) */
    else if (!el.checked && clearOnUncheck) {
        /* if clearOnUncheck is true, the value is cleared from the text-input,
           if clearOnUncheck is set to false, or not-set, the text-box value
           persists after unchecking the checkbox */
        targetEl.value = '';
    }
}

// references to the elements
var input2 = document.getElementById('input2'),
    select = document.getElementsByName('selectOne')[0],
    textInput = document.getElementById('displayTxtBox');

// binding the function to the onchange event of the input2 and selectOne elements.
input2.onchange = function() {
    showAmount(input2, select, textInput, true);
};
select.onchange = function() {
    /* because the clearOnUncheck argument depends on the changing of the
       checkbox there's no point in passing it to the showAmount() function
       in the select's onchange event-handler */
    showAmount(input2, select, textInput);
};​

参考资料：

. . . . . . . . 输入元素不能包含其他元素，因此它们没有结束标记：因此您的第一个输入是无效的HTML，这意味着浏览器尝试自动错误恢复，结果不可预测。请打开该文本，删除结束标记，并使用自动结束标记，而不是使用其他输入元素。另外，第二个输入标记的value属性有一个未关闭的字符串。