We will be building a simple kanban board in plain javascript , that can shuffle other task, so you can place the task at exact positions.

This is asked a lot of times in machine coding rounds for frontend interviews also.

Link : github.com/biohacker0/Kanban-Board

I am not discussing the html and css of the project, that you can see on the github.
We are focusing on the core logic.

Understanding the Setup:
We have draggable items (tasks) represented by HTML elements with the class name "task" and dropable areas (swimlanes) represented by HTML elements with the class name "swim-lane". We've attached event listeners to handle drag and drop events for these elements.

Base HTML Structure that is repeted , so we have swim-lane , which is the drop area driv.

And we have div's called task , there are things that we will drag , so these are our tasks.

<div class="swim-lane">
  <div class="task" draggable="true">Task 1</div>
  <div class="task" draggable="true">Task 2</div>
  <!-- More tasks -->
</div>
<!-- More swim-lanes -->

Breakdown of Key Components:

1. Retrieving Elements:

   • draggableItems = document.getElementsByClassName("task");

     • Selects all HTML elements with the class "task," which likely represent individual tasks.

   • dropableDivs = document.getElementsByClassName("swim-lane");

     • Selects all HTML elements with the class "swim-lane," which likely represent the different swim lanes or columns.

   •   let draggableItems = document.getElementsByClassName("task");
       let dropableDivs = document.getElementsByClassName("swim-lane");
     

2. Event Listeners for Tasks:

   • dragCb(event): When a task is dragged (dragstart event):

     • Adds the class "is-dragging" to the task, indicating that it's currently being dragged.

   • dragendCB(event): When dragging stops (dragend event):

     • Removes the "is-dragging" class, visually signifying the end of dragging.

    function dragendCB(event) {
      event.target.classList.remove("is-dragging");
    }

    function dragCb(event) {
      event.target.classList.add("is-dragging");
    }

    for (let task of draggableItems) {
      task.addEventListener("dragstart", dragCb);
      task.addEventListener("dragend", dragendCB);
    }

1. Drop Event Listener for Swim Lanes:

   • dragoverCB(event): When a dragged task is hovered over a swim lane (dragover event):

     • Prevents the default browser behavior, which might interfere with custom drag-and-drop logic.

     • Calls the helper function insertAboveTask to find the appropriate insertion point within the swim lane.

     • Retrieves the currently dragged task element using document.querySelector(".is-dragging").

     • If there's no task below the mouse pointer (meaning the task would be appended to the bottom):

       • Simply appends the dragged task to the swim lane using appendChild.

     • Otherwise, inserts the dragged task before the task that's closest below the mouse pointer using insertBefore.

         for (let dropableDiv of dropableDivs) {
           dropableDiv.addEventListener("dragover", function dragoverCB(event) {
             event.preventDefault();
             const bottomTask = insertAboveTask(dropableDiv, event.clientY);
             const currentTask = document.querySelector(".is-dragging");
       
             if (!bottomTask) dropableDiv.appendChild(currentTask);
             else dropableDiv.insertBefore(currentTask, bottomTask);
           });
         }
       

2. Finding the Insertion Point (insertAboveTask function):

   • Takes the swim lane element and the mouse Y coordinate as input.

   • Initializes variables to track the closest task and its offset from the mouse pointer.

   • Loops through all task elements within the swim lane:

     • Uses getBoundingClientRect() to get the task's top position.

     • Calculates the offset between the mouse pointer and the task's top position.

     • If the offset is negative (i.e., the mouse pointer is above the task) and closer than the current closest offset:

       • Updates the closest offset and closest task variables.

   • Returns the closest task, which will be used to determine the insertion point.

   • 

       function insertAboveTask(dropableDiv, mouseY) {
         const currentDropableDivTasks = dropableDiv.getElementsByClassName("task");
         let closestTask = null;
         let closestOffset = Number.NEGATIVE_INFINITY;
     
         for (task of currentDropableDivTasks) {
           const { top } = task.getBoundingClientRect();
           const offset = mouseY - top;
     
           if (offset < 0 && offset > closestOffset) {
             closestOffset = offset;
             closestTask = task;
           }
         }
     
         return closestTask;
       }
     

Core Logic to decide who is closest task to use :

Core Logic Breakdown

1. Event Listeners:

   • dragstart and dragend events are attached to draggable elements (tasks) to track the drag state and add/remove a visual indicator.

   • dragover event is attached to dropable elements (swimlanes) to handle the drag-and-drop process.

2. insertAboveTask Function:

   • Takes a dropableDiv and mouseY as input.

   • Iterates through all task elements within the dropableDiv.

   • Calculates the offset between the mouse's Y coordinate (mouseY) and the top of each task.

   • Finds the task with the smallest negative offset (closest to the mouse cursor above its current position).

   • If no suitable task is found (the mouse is above all tasks), the new task is appended to the end of the dropableDiv.

   • Otherwise, the new task is inserted before the identified task.

     

Understanding Offset Calculations

The insertAboveTask function's key logic lies in its offset calculations. Here's a breakdown:

1. offset = mouseY - top: This calculates the difference between the mouse's Y coordinate and the top edge of each task.

2. offset < 0 && offset > closestOffset: This ensures the offset is negative (mouse is above the task) and closer to 0 than any previously encountered offset, indicating it's the closest task above the mouse.

3. Finding the Closest Task: The loop iterates through all tasks, comparing offsets and updating closestTask if a closer one is found.

4. Insertion Decision: If no suitable closestTask is found (offset === Number.NEGATIVE_INFINITY), the new task is appended to the end. Otherwise, it's inserted before the closestTask.

Calculations of different scenarios :

• Sample dropableDiv with 3 tasks:

  • Task 1: top = 100px

  • Task 2: top = 200px

  • Task 3: top = 300px

• Mouse position (mouseY):

  • Case 1: mouseY = 150px

  • Case 2: mouseY = 250px

  • Case 3: mouseY = 350px

Calculations and Results:

• Case 1:

  • Loop iterates through tasks.

  • Task 1: offset = 50 (positive, ignored)

  • Task 2: offset = -50 (negative and closest, closestTask updated)

  • Task 3: offset = -150 (negative but further, ignored)

  • insertAboveTask returns Task 2 (dragged task inserted below).

• Case 2:

  • Similar logic, but closestTask will be Task 3 as offset is closer to 0.

• Case 3:

  • No negative offsets found, closestTask remains null, and the dragged task is appended to the end.

With this the core logic of kanban is done , yay