Drag and drop table column

Written byPhuoc Nguyen

#Basic setup

As mentioned in the Drag and drop element in a list example, we need handle three events:

`mousedown` for the all header cells, so user can click and drag the first cell in each column
`mousemove` for `document`: This event triggers when user moves the column around, and we will create and insert a placeholder column depending on the direction (left or right)
`mouseup` for `document`: This event occurs when user drags the column.

Here is the skeleton of these event handlers:

// Query the table
const table = document.getElementById('table');

const mouseDownHandler = function(e) {
    ...

    // Attach the listeners to `document`
    document.addEventListener('mousemove', mouseMoveHandler);
    document.addEventListener('mouseup', mouseUpHandler);
};

const mouseMoveHandler = function(e) {
    ...
};

const mouseUpHandler = function() {
    ...
    // Remove the handlers of `mousemove` and `mouseup`
    document.removeEventListener('mousemove', mouseMoveHandler);
    document.removeEventListener('mouseup', mouseUpHandler);
};

// Query all header cells
table.querySelectorAll('th').forEach(function(headerCell) {
    // Attach event handler
    headerCell.addEventListener('mousedown', mouseDownHandler);
});

#Clone the table when user is moving a column

Since this task is performed once, we need a flag to track if it's executed:

let isDraggingStarted = false;

const mouseMoveHandler = function(e) {
    if (!isDraggingStarted) {
        isDraggingStarted = true;

        cloneTable();
    }
    ...
};

`cloneTable` creates an element that has the same position as the table, and is shown right before the table:

let list;

const cloneTable = function () {
    // Get the bounding rectangle of table
    const rect = table.getBoundingClientRect();

    // Create new element
    list = document.createElement('div');

    // Set the same position as table
    list.style.position = 'absolute';
    list.style.left = `${rect.left}px`;
    list.style.top = `${rect.top}px`;

    // Insert it before the table
    table.parentNode.insertBefore(list, table);

    // Hide the table
    table.style.visibility = 'hidden';
};

Imagine that `list` consists of items which are cloned from the table columns:

const cloneTable = function() {
    ...

    // Get all cells
    const originalCells = [].slice.call(table.querySelectorAll('tbody td'));

    const originalHeaderCells = [].slice.call(table.querySelectorAll('th'));
    const numColumns = originalHeaderCells.length;

    // Loop through the header cells
    originalHeaderCells.forEach(function(headerCell, headerIndex) {
        const width = parseInt(window.getComputedStyle(headerCell).width);

        // Create a new table from given row
        const item = document.createElement('div');
        item.classList.add('draggable');

        const newTable = document.createElement('table');

        // Header
        const th = headerCell.cloneNode(true);
        let newRow = document.createElement('tr');
        newRow.appendChild(th);
        newTable.appendChild(newRow);

        const cells = originalCells.filter(function(c, idx) {
            return (idx - headerIndex) % numColumns === 0;
        });
        cells.forEach(function(cell) {
            const newCell = cell.cloneNode(true);
            newRow = document.createElement('tr');
            newRow.appendChild(newCell);
            newTable.appendChild(newRow);
        });

        item.appendChild(newTable);
        list.appendChild(item);
    });
};

After this step, we have the following `list`:

<!-- The list -->
<div>
    <!-- First item -->
    <div>
        <table>
            <!-- The first column of original table -->
            <tr>
                ...
            </tr>
            <tr>
                ...
            </tr>
            ...
        </table>
    </div>

    <!-- Second item -->
    <div>
        <table>
            <!-- The second column of original table -->
            <tr>
                ...
            </tr>
            <tr>
                ...
            </tr>
            ...
        </table>
    </div>

    <!-- ... -->
</div>

<!-- The original table -->
<table>
    ...
</table>

It's worth noting that when cloning cells in each item, we have to set the cell width same as the original cell. So the item looks like the original column completely:

originalHeaderCells.forEach(function(headerCell, headerIndex) {
    // Get the width of original cell
    const width = parseInt(window.getComputedStyle(headerCell).width);

    newTable.style.width = `${width}px`;

    cells.forEach(function(cell) {
        const newCell = cell.cloneNode(true);
        newCell.style.width = `${width}px`;
        ...
    });
});

#Determine the indexes of dragging and target columns

let draggingEle; // The dragging element
let draggingRowIndex; // The index of dragging column

const mouseDownHandler = function (e) {
    // Get the index of dragging column
    draggingColumnIndex = [].slice.call(table.querySelectorAll('th')).indexOf(e.target);
};

const mouseMoveHandler = function (e) {
    if (!isDraggingStarted) {
        cloneTable();

        // Query the dragging element
        draggingEle = [].slice.call(list.children)[draggingColumnIndex];
    }
};

const mouseUpHandler = function () {
    // Get the end index
    const endColumnIndex = [].slice.call(list.children).indexOf(draggingEle);
};

As we have `draggingColumnIndex` and `endColumnIndex`, it's now easy to check if user drops to the left or right of table. And we can decide how to move the target column before or after the dragging column:

const mouseUpHandler = function () {
    // Move the dragged column to `endColumnIndex`
    table.querySelectorAll('tr').forEach(function (row) {
        const cells = [].slice.call(row.querySelectorAll('th, td'));
        draggingColumnIndex > endColumnIndex
            ? cells[endColumnIndex].parentNode.insertBefore(cells[draggingColumnIndex], cells[endColumnIndex])
            : cells[endColumnIndex].parentNode.insertBefore(
                  cells[draggingColumnIndex],
                  cells[endColumnIndex].nextSibling
              );
    });
};

Following is the final demo. Try to drag and drop the first cell of any column.

#Demo

#See also

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Phước Nguyễn