Drag & Drop implementation using html/css and Javascript

Drag & drop is among the important technique a web developer must be comfortable working with. In this article we are going to cover important events involved in drag & drop. For better understanding, you need to be familiar with:

• html
• Javascript
• DOM events.

Drag & drop events can be categorised into two:

1. Events that happens on dragged element
2. Events that happens on target element(dropZone).

For these events to work, set draggable attribute in dragged element to true.

<p draggable="true" class="dragged-item">This is our draggable element</p>

NOTE: Images and links are draggable by default (No need to set draggable attribute to true)

1 Events that happen on dragged element

These are events that trigger when draggable elements moves from their original position to final position. The events are:

• dragstart event
• drag event
• dragend event

Now let's discuss each event to see when and how to use these events.

a) Dragstart event

The dragstart event triggers when user start to move dragged element.This event uses dataTransfer object to set data.

The dataTransfer object sets data using setData method. The setData method takes in two parameter:

1. format of data to be set ("text/plain", "text/html" etc)
2. data value (anything in string form)

e.dataTransfer.setData(format, data);

NOTE: The data set above, is later retrieved and used in drop event.

dragstart event implemention

1. Add dragstart event listener to dragged element

2. Get id of dragged element for later use (use event.target.getAttribute("id") to get id).

3. Set data(in text/plain format) to id of dragged element. The data is set usingsetData method in dataTransfer object.

   const draggedElement = document.querySelector(".dragged-item");
   draggedElement.addEventListener("dragstart",function(e){
    e.dataTansfer.setData("text/plain",e.target.id);
   })
   

   b) drag event

The drag event triggers when dragged element is moving. The drag event fires continously since it triggers when moving dragged element.

Drag event provides an opportunity to change appearence of dragged element so that users can visualize their action.

In drag event, change the appearance of dragged element using css.

drag event implementation

1. Add drag event listener to dragged element
2. Get dragged element(e.target)
3. Change the color of dragged element

draggedElement.addEventListener("drag",function(e){
    e.target.style.color = "grey";
})

NOTE:

You can ommit drag event if you not changing appearance of dragged element

It is possible to use setData method in dataTransfer object to set data in this drag event.

c) dragend event

The dragend event triggers when drag operation is completed. Drag operation can either be successful or unsuccessful.

This dragend event allows you to reset appearance of dragged element to initial appearance of dragged element.

dragend event implementation

1. Add dragend event listener to dragged element
2. Get dragged element(e.target)
3. Change the color of dragged element to its initial color

   draggedElement.addEventListener("drag",function(e){
    e.target.style.color = "grey";
   })
   

   NOTE: You can ommit dragend event if you not changing appearance of dragged element

2 Events that happen on drop target element

These are events that occur when dragged element moves over the drop target element(dropZone). The events modifies the dropZone element. The events are:

• dragenter event
• dragover event
• drop event
• dragleave event

let's discuss the above events and how they should be implemented.

a) dragenter event

Thedragenter event fires when dragged element enters the dropZone. This dragenter event provides a visual feedback that a dropZone element accepts dragged element(drop target).

dragenter event implemention

1. Create dropZone element in html

   <div class="drop-zone"></div>
   

2. Get dropZone element using selector

   const dropZone = document,querySelector(".drop-zone");
   

3. Add dragenter event listener to dropZone element
4. Using e.target, modify dropZone element(change bacground color).

   dropZone.addEventListener("dragenter",function(e){
   e.preventDefault()
   e.target.style.border = "dotted 4px black";
   })
   

   NOTE: The default behavior of dragenter is to avoid dropping of dragged element. The e.preventDefault() prevents the default behaviour of dragenter event hence allowing dropping of dragged element in dropZone.

b) dragover event

The dragover event fires when dragged element is over the dropZone. This dragover event fires every milliseconds.

The main purpose of dragover event is to prevent default. Preventing default, prevents dragged element from going back to its original position when handling drop event.

dragover event implemention

1. Get the dropZone element
2. Add dragover event listener to dropZone element
3. Prevent default of dragover event

dropZone.addEventListener("dragover",function(e){
e.preventDefault()
})

c) drop event

The drop event fires when dragged element drops in dropZone. The drop event is where we get the data that was set in dragstart event and use the data. The data is obtained from dataTransfer object using getData method.

drop event implemention

1. Get dropZone element
2. Add drop event listener to dropZone element
3. Prevent default of drop event (opening link when dropped)
4. Get the data that was set in dragstart event and store it in a variable called draggedElementId

   const draggedElementId = e.dataTransfer.getData("text/plain");
   

   NOTE: draggedElementId is a string (id of dragged element)

5. Get the dragged element using document.getElementById and store it in another variable called draggedElement

   const draggedElement = document.getElementById(draggedElementId);
   

6. Get the dropZone element(e.target) and append the draggedElement

    dropZone.addEventListener("drop",function(e){
        e.preventDefault()
        const draggedElementId = e.dataTransfer.getData("text/plain");
        const draggedElement = document.getElementById( draggedElementId);
        e.target.appendChild(draggedElement);
    })
   

   The draggedElement moves inside dropZone as child element.

d) dragleave event

The dragleave event fires when dragged element leaves the dropZone area. This dragleave event can be used to modify dropZone element to its original state( remove visual feedback in dragenter event).

dragleave event implementation

1. Get dropZone element
2. Add dragleave event listener to dropZone element
3. Reset dropZone element to its original state

dropZone.addEventListener("dragleave",function(e){
        e.preventDefault()
        e.target.style.border = "solid 4px black";
    })

Conclusion

In summary, mastering these drag & drop events is essential for creating interactive and user-friendly web applications. They allow you to build features like drag-and-drop interfaces, file uploads, and more.

By understanding and effectively implementing these events, you can create dynamic and user-friendly interfaces with drag & drop functionality, enhancing the user experience and interactivity of websites.