我正在从一个JavaScript数组创建一个下拉列表.我想在 Select 某些选项时显示范围.
例如，我想在 Select 了选项"a"和"c"而没有 Select "b"时显示跨度.
var city = ["a","b","c"];
// Add array to city dropdown
var select_city = document.getElementById("city");
for(var i = 0; i < city.length; i++) {
let opt = city[i];
let el = document.createElement("option");
el.textContent = opt;
el.value = opt;
select_city.appendChild(el);
};
// Display output
var output = document.getElementById("output");
select_city.onchange = function() {
output.style.display = (this.value == city.value && this.value == !"b") ? "block":"none";
};#output {
display: none;
}<select name="city" id="city">
<option value="-">- Choose city -</option>
</select>
<span id="output"></span>为什么这不管用？我怎样才能正确地做这件事？