Make a resizable element

Assume that we want to turn the following element to draggable element:

<div id="resizeMe" class="resizable">Resize me</div>

First, we need to prepare some elements that indicate the element is resizable. They are placed absolutely at the four sides of the original element. To make the demo simply, I only prepare two of them which are placed at the right and bottom sides:

<div id="resizeMe" class="resizable">
    Resize me
    <div class="resizer resizer-r"></div>
    <div class="resizer resizer-b"></div>
</div>

Here is the basic styles for the layout:

.resizable {
    position: relative;
}
.resizer {
    /* All resizers are positioned absolutely inside the element */
    position: absolute;
}

/* Placed at the right side */
.resizer-r {
    cursor: col-resize;
    height: 100%;
    right: 0;
    top: 0;
    width: 5px;
}

/* Placed at the bottom side */
.resizer-b {
    bottom: 0;
    cursor: row-resize;
    height: 5px;
    left: 0;
    width: 100%;
}

To make the element resizable, we are going to handle three events:

mousedown on the resizers: Track the current position of mouse and dimension of the original element
mousemove on document: Calculate how far the mouse has been moved, and adjust the dimension of the element
mouseup on document: Remove the event handlers of document

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

// The current position of mouse
let x = 0;
let y = 0;

// The dimension of the element
let w = 0;
let h = 0;

// Handle the mousedown event
// that's triggered when user drags the resizer
const mouseDownHandler = function (e) {
    // Get the current mouse position
    x = e.clientX;
    y = e.clientY;

    // Calculate the dimension of element
    const styles = window.getComputedStyle(ele);
    w = parseInt(styles.width, 10);
    h = parseInt(styles.height, 10);

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

const mouseMoveHandler = function (e) {
    // How far the mouse has been moved
    const dx = e.clientX - x;
    const dy = e.clientY - y;

    // Adjust the dimension of element
    ele.style.width = `${w + dx}px`;
    ele.style.height = `${h + dy}px`;
};

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

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

All the event handlers are ready. Finally, we attach the mousedown event handler to all the resizers:

// Query all resizers
const resizers = ele.querySelectorAll('.resizer');

// Loop over them
[].forEach.call(resizers, function (resizer) {
    resizer.addEventListener('mousedown', mouseDownHandler);
});

Make a resizable element

Demo

See also