Resize columns of a table

Assume that we want to resize any column of the following table:

<table id="resizeMe" class="table">
    ...
</table>

Prepare the resizer

For each column, we insert a div element indicating that the associated column can be resized. The resizer element is positioned absolutely inside the column. The CSS styles for them would be as below:

.table th {
    position: relative;
}
.resizer {
    /* Displayed at the right side of column */
    position: absolute;
    top: 0;
    right: 0;
    width: 5px;
    cursor: col-resize;
    user-select: none;
}

To create resizers and append them to columns, we have to query and loop over all columns:

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

// Query all headers
const cols = table.querySelectorAll('th');

// Loop over them
[].forEach.call(cols, function (col) {
    // Create a resizer element
    const resizer = document.createElement('div');
    resizer.classList.add('resizer');

    // Set the height
    resizer.style.height = `${table.offsetHeight}px`;

    // Add a resizer element to the column
    col.appendChild(resizer);

    // Will be implemented in the next section
    createResizableColumn(col, resizer);
});

Handle the resizer's events

We are going to implement a function, createResizableColumn, which accepts two parameters:

col that represents the table header
resizer that represents the resizer element within the column

In order to allow user to resize col, we have to handle three events:

mousedown on the resizer: Track the current position of mouse
mousemove on document: Calculate how far the mouse has been moved, and adjust the width of the column
mouseup on document: Remove the event handlers of document

const createResizableColumn = function (col, resizer) {
    // Track the current position of mouse
    let x = 0;
    let w = 0;

    const mouseDownHandler = function (e) {
        // Get the current mouse position
        x = e.clientX;

        // Calculate the current width of column
        const styles = window.getComputedStyle(col);
        w = parseInt(styles.width, 10);

        // Attach listeners for document's events
        document.addEventListener('mousemove', mouseMoveHandler);
        document.addEventListener('mouseup', mouseUpHandler);
    };

    const mouseMoveHandler = function (e) {
        // Determine how far the mouse has been moved
        const dx = e.clientX - x;

        // Update the width of column
        col.style.width = `${w + dx}px`;
    };

    // When user releases the mouse, remove the existing event listeners
    const mouseUpHandler = function () {
        document.removeEventListener('mousemove', mouseMoveHandler);
        document.removeEventListener('mouseup', mouseUpHandler);
    };

    resizer.addEventListener('mousedown', mouseDownHandler);
};

Tip
This post uses the Attach event handlers inside other handlers tip

Highlight the resizer

We can improve the user experience a little bit. When user hovers or clicks on the resizer, it can be hightlighted. To demonstrate the idea in the most simple way, we add a solid border to the :hover selector:

.resizer:hover,
.resizing {
    border-right: 2px solid blue;
}

The resizing class is added to the resizer while user clicks and drags the resizer:

const mouseDownHandler = function(e) {
    ...
    resizer.classList.add('resizing');
};

const mouseUpHandler = function() {
    ...
    resizer.classList.remove('resizing');
};

Resize columns of a table

Prepare the resizer

Handle the resizer's events

Highlight the resizer

Demo

See also