使用javascript自动选择自定义选择框中的第一个选项

Question

I'm trying to create a customized select box and I found a great site that offered the necessary code that fits my requirements. But there is one thing that is missing from the code and that is to automatically select the first option in the list when I click on the dropdown box. In other words, I would like the first item from the list highlighted when I click on the arrow. I would really appreciate if somebody can help me add this functionality into the code.

The site where I found the code is from W3Schools and the link is

https://www.w3schools.com/howto/tryit.asp?filename=tryhow_custom_select

Thanks in advance!

HTML

---

    <div class="custom-select" style="width:200px;">
  <select>
    <option value="0">Select car:</option>
    <option value="1">Audi</option>
    <option value="2">BMW</option>
    <option value="3">Citroen</option>
    <option value="4">Ford</option>
    <option value="5">Honda</option>
    <option value="6">Jaguar</option>
    <option value="7">Land Rover</option>
    <option value="8">Mercedes</option>
    <option value="9">Mini</option>
    <option value="10">Nissan</option>
    <option value="11">Toyota</option>
    <option value="12">Volvo</option>
  </select>
</div>

CSS

---

/*the container must be positioned relative:*/
.custom-select {
  position: relative;
  font-family: Arial;
}
.custom-select select {
  display: none; /*hide original SELECT element:*/
}
.select-selected {
  background-color: DodgerBlue;
}
/*style the arrow inside the select element:*/
.select-selected:after {
  position: absolute;
  content: "";
  top: 14px;
  right: 10px;
  width: 0;
  height: 0;
  border: 6px solid transparent;
  border-color: #fff transparent transparent transparent;
}
/*point the arrow upwards when the select box is open (active):*/
.select-selected.select-arrow-active:after {
  border-color: transparent transparent #fff transparent;
  top: 7px;
}
/*style the items (options), including the selected item:*/
.select-items div,.select-selected {
  color: #ffffff;
  padding: 8px 16px;
  border: 1px solid transparent;
  border-color: transparent transparent rgba(0, 0, 0, 0.1) transparent;
  cursor: pointer;
}
/*style items (options):*/
.select-items {
  position: absolute;
  background-color: DodgerBlue;
  top: 100%;
  left: 0;
  right: 0;
  z-index: 99;
}
/*hide the items when the select box is closed:*/
.select-hide {
  display: none;
}
.select-items div:hover, .same-as-selected {
  background-color: rgba(0, 0, 0, 0.1);
}

Javascript

---

var x, i, j, selElmnt, a, b, c;
/*look for any elements with the class "custom-select":*/
x = document.getElementsByClassName("custom-select");
for (i = 0; i < x.length; i++) {
  selElmnt = x[i].getElementsByTagName("select")[0];
  /*for each element, create a new DIV that will act as the selected item:*/
  a = document.createElement("DIV");
  a.setAttribute("class", "select-selected");
  a.innerHTML = selElmnt.options[selElmnt.selectedIndex].innerHTML;
  x[i].appendChild(a);
  /*for each element, create a new DIV that will contain the option list:*/
  b = document.createElement("DIV");
  b.setAttribute("class", "select-items select-hide");
  for (j = 1; j < selElmnt.length; j++) {
    /*for each option in the original select element,
    create a new DIV that will act as an option item:*/
    c = document.createElement("DIV");
    c.innerHTML = selElmnt.options[j].innerHTML;
    c.addEventListener("click", function(e) {
        /*when an item is clicked, update the original select box,
        and the selected item:*/
        var y, i, k, s, h;
        s = this.parentNode.parentNode.getElementsByTagName("select")[0];
        h = this.parentNode.previousSibling;
        for (i = 0; i < s.length; i++) {
          if (s.options[i].innerHTML == this.innerHTML) {
            s.selectedIndex = i;
            h.innerHTML = this.innerHTML;
            y = this.parentNode.getElementsByClassName("same-as-selected");
            for (k = 0; k < y.length; k++) {
              y[k].removeAttribute("class");
            }
            this.setAttribute("class", "same-as-selected");
            break;
          }
        }
        h.click();
    });
    b.appendChild(c);
  }
  x[i].appendChild(b);
  a.addEventListener("click", function(e) {
      /*when the select box is clicked, close any other select boxes,
      and open/close the current select box:*/
      e.stopPropagation();
      closeAllSelect(this);
      this.nextSibling.classList.toggle("select-hide");
      this.classList.toggle("select-arrow-active");
  });
}
function closeAllSelect(elmnt) {
  /*a function that will close all select boxes in the document,
  except the current select box:*/
  var x, y, i, arrNo = [];
  x = document.getElementsByClassName("select-items");
  y = document.getElementsByClassName("select-selected");
  for (i = 0; i < y.length; i++) {
    if (elmnt == y[i]) {
      arrNo.push(i)
    } else {
      y[i].classList.remove("select-arrow-active");
    }
  }
  for (i = 0; i < x.length; i++) {
    if (arrNo.indexOf(i)) {
      x[i].classList.add("select-hide");
    }
  }
}
/*if the user clicks anywhere outside the select box,
then close all select boxes:*/
document.addEventListener("click", closeAllSelect);

Answer 1

In order to select an option in a dropdown list by default, all you need to do is apply the selected attribute to the desired <option> . For example,

<option value="1" selected>Audi</option>

This can be seen in the following:

 var x, i, j, selElmnt, a, b, c; /*look for any elements with the class "custom-select":*/ x = document.getElementsByClassName("custom-select"); for (i = 0; i < x.length; i++) { selElmnt = x[i].getElementsByTagName("select")[0]; /*for each element, create a new DIV that will act as the selected item:*/ a = document.createElement("DIV"); a.setAttribute("class", "select-selected"); a.innerHTML = selElmnt.options[selElmnt.selectedIndex].innerHTML; x[i].appendChild(a); /*for each element, create a new DIV that will contain the option list:*/ b = document.createElement("DIV"); b.setAttribute("class", "select-items select-hide"); for (j = 1; j < selElmnt.length; j++) { /*for each option in the original select element, create a new DIV that will act as an option item:*/ c = document.createElement("DIV"); c.innerHTML = selElmnt.options[j].innerHTML; c.addEventListener("click", function(e) { /*when an item is clicked, update the original select box, and the selected item:*/ var y, i, k, s, h; s = this.parentNode.parentNode.getElementsByTagName("select")[0]; h = this.parentNode.previousSibling; for (i = 0; i < s.length; i++) { if (s.options[i].innerHTML == this.innerHTML) { s.selectedIndex = i; h.innerHTML = this.innerHTML; y = this.parentNode.getElementsByClassName("same-as-selected"); for (k = 0; k < y.length; k++) { y[k].removeAttribute("class"); } this.setAttribute("class", "same-as-selected"); break; } } h.click(); }); b.appendChild(c); } x[i].appendChild(b); a.addEventListener("click", function(e) { /*when the select box is clicked, close any other select boxes, and open/close the current select box:*/ e.stopPropagation(); closeAllSelect(this); this.nextSibling.classList.toggle("select-hide"); this.classList.toggle("select-arrow-active"); }); } function closeAllSelect(elmnt) { /*a function that will close all select boxes in the document, except the current select box:*/ var x, y, i, arrNo = []; x = document.getElementsByClassName("select-items"); y = document.getElementsByClassName("select-selected"); for (i = 0; i < y.length; i++) { if (elmnt == y[i]) { arrNo.push(i) } else { y[i].classList.remove("select-arrow-active"); } } for (i = 0; i < x.length; i++) { if (arrNo.indexOf(i)) { x[i].classList.add("select-hide"); } } } /*if the user clicks anywhere outside the select box, then close all select boxes:*/ document.addEventListener("click", closeAllSelect); 

 /*the container must be positioned relative:*/ .custom-select { position: relative; font-family: Arial; } .custom-select select { display: none; /*hide original SELECT element:*/ } .select-selected { background-color: DodgerBlue; } /*style the arrow inside the select element:*/ .select-selected:after { position: absolute; content: ""; top: 14px; right: 10px; width: 0; height: 0; border: 6px solid transparent; border-color: #fff transparent transparent transparent; } /*point the arrow upwards when the select box is open (active):*/ .select-selected.select-arrow-active:after { border-color: transparent transparent #fff transparent; top: 7px; } /*style the items (options), including the selected item:*/ .select-items div, .select-selected { color: #ffffff; padding: 8px 16px; border: 1px solid transparent; border-color: transparent transparent rgba(0, 0, 0, 0.1) transparent; cursor: pointer; } /*style items (options):*/ .select-items { position: absolute; background-color: DodgerBlue; top: 100%; left: 0; right: 0; z-index: 99; } /*hide the items when the select box is closed:*/ .select-hide { display: none; } .select-items div:hover, .same-as-selected { background-color: rgba(0, 0, 0, 0.1); } 

 <div class="custom-select" style="width:200px;"> <select> <option value="0">Select car:</option> <option value="1" selected>Audi</option> <option value="2">BMW</option> <option value="3">Citroen</option> <option value="4">Ford</option> <option value="5">Honda</option> <option value="6">Jaguar</option> <option value="7">Land Rover</option> <option value="8">Mercedes</option> <option value="9">Mini</option> <option value="10">Nissan</option> <option value="11">Toyota</option> <option value="12">Volvo</option> </select> </div> 

Answer 2

希望有帮助，请尝试禁用“选择汽车”元素

<option value="0" disabled>Select car:</option>

Answer 3

您可以将所选内容放入选项<option value="1" selected>Audi</option>这将是默认选择。