function autocomplete(inp, arr) {
 /*函數主要有兩個參數：文本框元素和自動補齊的完整數據*/
 var currentFocus;
 /* 監聽 - 在寫入時觸發 */
 inp.addEventListener("input", function(e) {
   var a, b, i, val = this.value;
   /*關閉已經打開的自動完成值列表*/
   closeAllLists();
   if (!val) { return false;}
   currentFocus = -1;
   /*創建列表*/
   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);
   /*循環數組...*/
   for (i = 0; i < arr.length; i++) {
    /*檢查選項是否以與文本字段值相同的字母開頭*/
    if (arr[i].substr(0, val.length).toUpperCase() == val.toUpperCase()) {
     /*為匹配元素創建 DIV*/
     b = document.createElement("DIV");
     /*使匹配字母變粗體*/
     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);
   });
}

/*數組 - 包含所有國家或地區名*/
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"];

/*傳遞參數*/
autocomplete(document.getElementById("myInput"), countries);

* { 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; 
}

<!-- 關閉自帶的自動補全功能 -->
<form autocomplete="off" action="/action_page.php">
 <div class="autocomplete" >
  <input id="myInput" type="text" name="myCountry" placeholder="輸入國家或地區英文名...">
 </div>
 <input type="submit">
</form>