<!-- 关闭自带的自动补全功能 -->
<form autocomplete="off" action="/action_page.php">
    <div class="autocomplete" style="width:300px;">
        <input id="myInput" type="text" name="myCountry" placeholder="输入国家或地区英文名...">
    </div>
    <input type="submit">
</form>
<style>
    * { box-sizing: border-box; }
    body {
      font: 16px Arial; 
    }
    .autocomplete {
      /*the container must be positioned relative:*/
      position: relative;
      display: inline-block;
    }
    input {
      border: 1px solid transparent;
      background-color: #f1f1f1;
      padding: 10px;
      font-size: 16px;
    }
    input[type=text] {
      background-color: #f1f1f1;
      width: 100%;
    }
    input[type=submit] {
      background-color: DodgerBlue;
      color: #fff;
    }
    .autocomplete-items {
      position: absolute;
      border: 1px solid #d4d4d4;
      border-bottom: none;
      border-top: none;
      z-index: 99;
      /*position the autocomplete items to be the same width as the container:*/
      top: 100%;
      left: 0;
      right: 0;
    }
    .autocomplete-items div {
      padding: 10px;
      cursor: pointer;
      background-color: #fff; 
      border-bottom: 1px solid #d4d4d4; 
    }
    .autocomplete-items div:hover {
      /*when hovering an item:*/
      background-color: #e9e9e9; 
    }
    .autocomplete-active {
      /*when navigating through the items using the arrow keys:*/
      background-color: DodgerBlue !important; 
      color: #ffffff; 
    }
</style>
<script>
    function autocomplete(inp, arr) {
      /*函数主要有两个参数：文本框元素和自动补齐的完整数据*/
      var currentFocus;
      /* 监听 - 在写入时触发 */
      inp.addEventListener("input", function(e) {
          var a, b, i, val = this.value;
          /*关闭已经打开的自动完成值列表*/
          closeAllLists();
          if (!val) { return false;}
          currentFocus = -1;
          /*创建列表*/
          a = document.createElement("DIV");
          a.setAttribute("id", this.id + "autocomplete-list");
          a.setAttribute("class", "autocomplete-items");
          /*添加 DIV 元素*/
          this.parentNode.appendChild(a);
          /*循环数组...*/
          for (i = 0; i < arr.length; i++) {
            /*检查选项是否以与文本字段值相同的字母开头*/
            if (arr[i].substr(0, val.length).toUpperCase() == val.toUpperCase()) {
              /*为匹配元素创建 DIV*/
              b = document.createElement("DIV");
              /*使匹配字母变粗体*/
              b.innerHTML = "<strong>" + arr[i].substr(0, val.length) + "</strong>";
              b.innerHTML += arr[i].substr(val.length);
              /*insert a input field that will hold the current array item's value:*/
              b.innerHTML += "<input type='hidden' value='" + arr[i] + "'>";
              /*execute a function when someone clicks on the item value (DIV element):*/
              b.addEventListener("click", function(e) {
                  /*insert the value for the autocomplete text field:*/
                  inp.value = this.getElementsByTagName("input")[0].value;
                  /*close the list of autocompleted values,
                  (or any other open lists of autocompleted values:*/
                  closeAllLists();
              });
              a.appendChild(b);
            }
          }
      });
      /*execute a function presses a key on the keyboard:*/
      inp.addEventListener("keydown", function(e) {
          var x = document.getElementById(this.id + "autocomplete-list");
          if (x) x = x.getElementsByTagName("div");
          if (e.keyCode == 40) {
            /*If the arrow DOWN key is pressed,
            increase the currentFocus variable:*/
            currentFocus++;
            /*and and make the current item more visible:*/
            addActive(x);
          } else if (e.keyCode == 38) { //up
            /*If the arrow UP key is pressed,
            decrease the currentFocus variable:*/
            currentFocus--;
            /*and and make the current item more visible:*/
            addActive(x);
          } else if (e.keyCode == 13) {
            /*If the ENTER key is pressed, prevent the form from being submitted,*/
            e.preventDefault();
            if (currentFocus > -1) {
              /*and simulate a click on the "active" item:*/
              if (x) x[currentFocus].click();
            }
          }
      });
      function addActive(x) {
        /*a function to classify an item as "active":*/
        if (!x) return false;
        /*start by removing the "active" class on all items:*/
        removeActive(x);
        if (currentFocus >= x.length) currentFocus = 0;
        if (currentFocus < 0) currentFocus = (x.length - 1);
        /*add class "autocomplete-active":*/
        x[currentFocus].classList.add("autocomplete-active");
      }
      function removeActive(x) {
        /*a function to remove the "active" class from all autocomplete items:*/
        for (var i = 0; i < x.length; i++) {
          x[i].classList.remove("autocomplete-active");
        }
      }
      function closeAllLists(elmnt) {
        /*close all autocomplete lists in the document,
        except the one passed as an argument:*/
        var x = document.getElementsByClassName("autocomplete-items");
        for (var i = 0; i < x.length; i++) {
          if (elmnt != x[i] && elmnt != inp) {
            x[i].parentNode.removeChild(x[i]);
          }
        }
      }
      /*execute a function when someone clicks in the document:*/
      document.addEventListener("click", function (e) {
          closeAllLists(e.target);
          });
    }
    
    /*数组 - 包含所有国家或地区名*/
    var countries = ["Afghanistan","Albania","Algeria","Andorra","Angola","Anguilla","Antigua & Barbuda","Argentina","Armenia","Aruba","Australia","Austria","Azerbaijan","Bahamas","Bahrain","Bangladesh","Barbados","Belarus","Belgium","Belize","Benin","Bermuda","Bhutan","Bolivia","Bosnia & Herzegovina","Botswana","Brazil","British Virgin Islands","Brunei","Bulgaria","Burkina Faso","Burundi","Cambodia","Cameroon","Canada","Cape Verde","Cayman Islands","Central Arfrican Republic","Chad","Chile","China","Colombia","Congo","Cook Islands","Costa Rica","Cote D Ivoire","Croatia","Cuba","Curacao","Cyprus","Czech Republic","Denmark","Djibouti","Dominica","Dominican Republic","Ecuador","Egypt","El Salvador","Equatorial Guinea","Eritrea","Estonia","Ethiopia","Falkland Islands","Faroe Islands","Fiji","Finland","France","French Polynesia","French West Indies","Gabon","Gambia","Georgia","Germany","Ghana","Gibraltar","Greece","Greenland","Grenada","Guam","Guatemala","Guernsey","Guinea","Guinea Bissau","Guyana","Haiti","Honduras","Hong Kong China","Hungary","Iceland","India","Indonesia","Iran","Iraq","Ireland","Isle of Man","Israel","Italy","Jamaica","Japan","Jersey","Jordan","Kazakhstan","Kenya","Kiribati","Kosovo","Kuwait","Kyrgyzstan","Laos","Latvia","Lebanon","Lesotho","Liberia","Libya","Liechtenstein","Lithuania","Luxembourg","Macau China","Macedonia","Madagascar","Malawi","Malaysia","Maldives","Mali","Malta","Marshall Islands","Mauritania","Mauritius","Mexico","Micronesia","Moldova","Monaco","Mongolia","Montenegro","Montserrat","Morocco","Mozambique","Myanmar","Namibia","Nauro","Nepal","Netherlands","Netherlands Antilles","New Caledonia","New Zealand","Nicaragua","Niger","Nigeria","North Korea","Norway","Oman","Pakistan","Palau","Palestine","Panama","Papua New Guinea","Paraguay","Peru","Philippines","Poland","Portugal","Puerto Rico","Qatar","Reunion","Romania","Russia","Rwanda","Saint Pierre & Miquelon","Samoa","San Marino","Sao Tome and Principe","Saudi Arabia","Senegal","Serbia","Seychelles","Sierra Leone","Singapore","Slovakia","Slovenia","Solomon Islands","Somalia","South Africa","South Korea","South Sudan","Spain","Sri Lanka","St Kitts & Nevis","St Lucia","St Vincent","Sudan","Suriname","Swaziland","Sweden","Switzerland","Syria","Taiwan China","Tajikistan","Tanzania","Thailand","Timor L'Este","Togo","Tonga","Trinidad & Tobago","Tunisia","Turkey","Turkmenistan","Turks & Caicos","Tuvalu","Uganda","Ukraine","United Arab Emirates","United Kingdom","United States of America","Uruguay","Uzbekistan","Vanuatu","Vatican City","Venezuela","Vietnam","Virgin Islands (US)","Yemen","Zambia","Zimbabwe"];
    
    /*传递参数*/
    autocomplete(document.getElementById("myInput"), countries);
</script>