A Local Kanban Board With HTML5 Drag and Drop and Immutable Board State

The board state is a plain tree object: board → columns → cards. All 12 mutation functions return a new tree without touching the input. HTML5 drag-and-drop provides the interaction layer — no library needed, but the event model has a few gotchas worth knowing about.

Every team uses Kanban, but the cloud versions cost money and require accounts. A local board that lives entirely in your browser — stored in localStorage, exportable as JSON — is surprisingly serviceable for personal projects.

🔗 Live demo: https://sen.ltd/portfolio/kanban-local/
📦 GitHub: https://github.com/sen-ltd/kanban-local

Features:

Multiple boards with custom columns
HTML5 drag and drop (cards between columns, reorder within column, reorder columns)
localStorage persistence
JSON import / export
Search / filter across all cards
Color labels per card
Japanese / English UI
Zero dependencies, 46 tests

Immutable state

The whole app state is one object tree. Every operation returns a new tree:

export function addCard(board, columnId, card) {
  return {
    ...board,
    columns: board.columns.map(col =>
      col.id === columnId ? { ...col, cards: [...col.cards, card] } : col
    ),
  };
}

export function moveCard(board, fromColId, cardId, toColId, toIndex) {
  const fromCol = board.columns.find(c => c.id === fromColId);
  const card = fromCol.cards.find(c => c.id === cardId);
  return {
    ...board,
    columns: board.columns.map(col => {
      if (col.id === fromColId) {
        return { ...col, cards: col.cards.filter(c => c.id !== cardId) };
      }
      if (col.id === toColId) {
        const newCards = [...col.cards];
        newCards.splice(toIndex, 0, card);
        return { ...col, cards: newCards };
      }
      return col;
    }),
  };
}

fromCol === toCol gets handled by a separate reorderCard path because both branches need to apply in sequence.

The same-column drop index adjustment

When dragging within a single column, there's an off-by-one to handle: if you remove the dragged card first, the destination index shifts. The UI layer handles this:

function handleDrop(fromColId, cardId, toColId, rawIndex) {
  let toIndex = rawIndex;
  if (fromColId === toColId) {
    const fromIndex = findCardIndex(fromColId, cardId);
    if (toIndex > fromIndex) toIndex -= 1;
  }
  state = moveCard(state, fromColId, cardId, toColId, toIndex);
}

Without this, dragging a card from position 0 to position 2 would end up at position 1, because the removal of position 0 shifts everything down.

HTML5 drag events

The native drag API has a specific event sequence:

dragstart on source → set dataTransfer, add a CSS class
dragenter on target → first time cursor enters
dragover on target → fires continuously while hovering. Must preventDefault() or drop won't work.
dragleave on target → cursor leaves (fires on child element boundaries too — watch out)
drop on target → set final state
dragend on source → always fires, even on cancel. Use this for cleanup.

card.addEventListener('dragstart', (e) => {
  e.dataTransfer.setData('text/plain', cardId);
  card.classList.add('dragging');
});

column.addEventListener('dragover', (e) => {
  e.preventDefault(); // CRITICAL
  // Compute drop index based on cursor Y position
});

column.addEventListener('drop', (e) => {
  e.preventDefault();
  const cardId = e.dataTransfer.getData('text/plain');
  // apply move
});

The dragleave firing on child boundaries is a common gotcha. If you want to remove the highlight when the cursor genuinely leaves the column, you need to track enter/leave counts or use e.relatedTarget checks.

localStorage persistence

Every state mutation triggers a save:

function saveState() {
  localStorage.setItem('kanban-state', JSON.stringify({ boards, activeBoard: boardId }));
}

The whole app state is small (kilobytes even with many cards), so serializing on every change is fine. For apps that could grow large, you'd debounce or diff — but here, immediate saves keep the UX simple.