如何通过选择复选框和下拉列表用Javascript更改标签值

给定更正的HTML，添加id属性和label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="" />​

以下JavaScript似乎对您的预期结果有效，尽管我并不完全确定您的意图：

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);
};​

JS Fiddle演示与clearOnUncheck == true

带有clearOnUncheck == false 的JS Fiddle演示

带有clearOnUncheck的JS Fiddle演示未设置

参考文献：

• checked
• document.getElementById()
• document.getElementsByName()
• document.getElementsByTagName()
• CCD_ 10
• CCD_ 11
• 三元/条件运算符
• typeof