How to create an Autocompletion functionality with CSS , Javascript and HTML ?

The input box

Below we have created the input box with the HTML input element of text type (we may have also use the search input box)

<input 
    id="color-control" 
    type="search" 
    name="color" 
    placeholder="Type a color" 
    autocomplete="off"
>

The autocompletion list

The popover contains the autocompletion list

It will be created and refreshed for every value change.

• In html, it will looks like that:

<ul id="color-autocompletion-list">
    <li>Blue</li>
    <li>Red</a></li>
    <li>Yellow</li>
    <li>...</li>
</ul>

• The styling (by default a list has a bullet and a margin that we delete)

 
#color-autocompletion-list {
  padding: 0;
  list-style-type: none;
  border: 1px solid rgba(0,0,0,.125);
  border-radius: .25rem;
  width: fit-content;
}
#color-autocompletion-list li {
   padding: 1rem;
}
#color-autocompletion-list li:hover {
    color: #fff;
    background-color: #6c757d;
    border-color: #6c757d;
}

• In Javascript, we would could create the above completion list with the following function that uses javascript regular expression to filter the data

/* The possible values */
let colors = ["Blue","Red","Yellow"];

/**
  The function that creates the autocompletion list with:
    * a search term as parameter
    * the listElement that receives the list item
    * the inputBox to receive the value when an element is clicked
*/
function addAutoCompletionListItem(searchTerm, listElement, inputBox){
   console.log(`filtering value ${searchTerm}`);
   /* Reset the content */
   listElement.innerHtml=""
   /* No search term */
   if(searchTerm==""){
      return;
   }
   /* Filtering of the data */
   let pattern = new RegExp(`.*${searchTerm}.*`,"i");
   let selectedColors = colors.filter(e=>pattern.test(e));
   if (selectedColors.length==0){
     return;
   }
   /* Creation of the above autocompletion list item */
   for(color of selectedColors){
        let li = document.createElement("li");
        /* To be able to determine that an item was clicked */
        li.classList.add("color-autocompletion-list-item");
        /* We make li focusable to get on the relatedTarget it when we click  */
        li.setAttribute("tabindex","0");
        li.innerText = color;
        /* When the element is clicked, set the value of the search Box */
        li.addEventListener("click", function(){;
             if (inputBox != undefined){
                /* Set the value in the input box */
                inputBox.value = this.innerText;
                /* Remove the list */
                listElement.remove();
             } else {
               console.log(`No input box defined, the selected value was ${this.innerText}`);
             }
        });
        listElement.append(li);
   }
   return listElement;
}

• To test, we filter with the dot star to get all values and append it to the body

let completionList = document.createElement("ul");
completionList.setAttribute("id","color-autocompletion-list");
completionList = addAutoCompletionListItem(".*", completionList);
document.body.insertAdjacentElement('afterbegin', completionList);

• Output: Hover over the list and click on an element

The positioning of the autocompletion list

To position the list at the bottom of the input box, we will use the relative absolute positioning where:

• the input box is inside a container
• the list is relatively positioned at the bottom of this container

Making the list CSS - Absolute Positioning (Relative / Absolute / Fixed > Absolute or Static > Absolute ), make the list an overlay

• We add a div container element and make it relative. Therefore all inner absolute positioned element coordinates will be based on this box

<p>A little bit of text to show that the list follows its target input box</p>
<div style="position:relative">
    <input id="color-control"  type="search" name="color" placeholder="Type a color"  autocomplete="off">
    <ul id="color-autocompletion-list">
        <li>Blue</li>
        <li>Red</a></li>
        <li>Yellow</li>
        <li>...</li>
    </ul>
</div>
<p>A text that should be covered to show the overlay functionality</p>

#color-autocompletion-list {
  position: absolute; 
  margin: 0px; 
  background-color: white;
}

The display of the autocompletion list

The previous autocompletion list should be:

• shown when the input element has focus
• deleted when the input element has lose focus (ie blur

This section shows you how.

• The below Javascript code shows how we:
  • create the autocompletion list on focus
  • add it with insertAdjacent as a sibling of the input box
  • delete the autocompletion list on blur

/* The input element */
let inputBox = document.getElementById("color-input");

/* The list */
let completionList = document.createElement("ul");
completionList.setAttribute("id","color-autocompletion-list");

/* on focus, create the autocompletion list */
inputBox.addEventListener("focus", function(){
    
    /* on focus, create the list */
    completionList  = setAutoCompletionListItem(".*",completionList ,this);
    inputBox.insertAdjacentElement('afterend',completionList);
    
    /* on blur, delete it */
    inputBox.addEventListener("blur", function(event){
        /* blur can also happen when we click on a list item, we don't remove the list in this case */
        if (event.relatedTarget == null ||
           (event.relatedTarget  !=null && !event.relatedTarget.classList.contains("color-autocompletion-list-item"))){
           completionList.innerHTML = "";
           completionList.remove();
        }
    });
});

• Result:
  • Click on the input box, you will see the whole list
    • Select an item, the value would go into the input box
    • Or click to the right, the auto-completion list would disappear

The filtering of the autocompletion list

To filter the values of the autocompletion list when a user enters a value into the input box, we need to use the input event.

This event fires each time the input value changes.

/* The input element */
var inputBox = document.getElementById("color-input");

/* The list */
var completionList = document.createElement("ul");
completionList.setAttribute("id","color-autocompletion-list");

/* on the input event, create and filter the autocompletion list with the value */
inputBox.addEventListener("input", function(){
    let searchTerm = this.value;
    if (this.value==""){
       searchTerm = ".*";
    }
    completionList = setAutoCompletionListItem(searchTerm,completionList,this);
});

• Result:
  • Enter a value in the input box and see the filtering happening
  • Delete the value or take focus to see the whole list again

Final thoughts and Library

This is it, you got all elements of an auto-completion list. You can refactor it:

• by creating a object
• by adding or removing functionality
• by getting your data from a server
• en so on…

And if you don't want to develop your own, your can always use a library.

• Full:
  • auto-complete
  • https://www.downshift-js.com/
  • See also the search box library
• Component:
  • Floating UI
  • popper for positioning
  • Bootstrap dropdown for positioning (popper based) and container