# Resizable Div Box with Mouse Dragging Support Using Vanilla JavaScript / TypeScript

September 06, 2020

Recently, I have created a Demo Project, which demonstrates how to create a resizable div box by using mouse clicking and dragging. It uses vanilla TypeScript (or removing those types to be JavaScript). I created this Demo project simply because I wanted to explore how to achieve this effects purely by manipulating with different div elements and event listeners. Here you go:

There are some concepts behind this demo, which I would like to mention

# Determine The Position of the Box

Well, this is a simple math implementation here.

To draw a resizable box, you need to do a mouse click firstly, move the mouse without releasing it, and lastly release the mouse in the end. Hence, there are 2 points, the first point being the point where you hit the mouse and the second point being the point where you release the mouse.

Then, the straight line between the 2 points is how your mouse has moved. And from this line, you can do some calculations and find out the position of the div box and its width and height.

As illustrated in the following diagram, point start and point end both has individual information of cursor events with their `x` and `y` values.

Assume point start is `(X1, Y1)` and point end is `(X2, Y2)`.

You can use

• `abs(X2 - X1)` to get width
• `abs(Y2 - Y1)` to get height
• `min(Y1, Y2)` to get top
• `min(X1, X2)` to get left

And then, all you have to do is just applying these numbers to the respective CSS styles of the div.

```refresh() { const left = Math.min(this.startCoordinator.x, this.endCoordinator.x); const right = Math.max(this.startCoordinator.x, this.endCoordinator.x); const top = Math.min(this.startCoordinator.y, this.endCoordinator.y); const bottom = Math.max(this.startCoordinator.y, this.endCoordinator.y); const height = bottom - top; const width = right - left; this.ref.style.top = `\${top}px`; this.ref.style.left = `\${left}px`; this.ref.style.width = `\${width}px`; this.ref.style.height = `\${height}px`; }```

Every time when any of `top`, `left`, `height` and `width` changes, it indicates the div box should be refreshed and repainted in the web page. By achieving that, simply use the `refresh()` function to update the styles of the div box

# Resize the Box Using 8 Adjusters

After initially drawing the div box, there should be 8 adjusters to allow users to resize the box.

The 8 adjusters are named as

• `TOP`
• `BOTTOM`
• `LEFT`
• `RIGHT`
• `TOP_LEFT`
• `TOP_RIGHT`
• `BOTTOM_LEFT`
• `BOTTOM_RIGHT`

Again, based on the mouse click starting point and ending point, we can get the movement value of X and Y. And with the position of the adjuster, we can know how to resize the box correctly.

For example, For `Top_Left` adjuster,

```case Position.TOP_LEFT: top += movementY; height -= movementY; left += movementX; width -= movementX; break;```

Then we can get the updated `top`, `left`, `height` and `width`.

And then based on these values, we can then re-calculate the starting point and the ending point, then refresh and repaint the div box on the web page.

```this.startCoordinator = { x: left, y: top }; this.endCoordinator = { x: left + width, y: top + height }; this.ref.style.top = `\${top}px`; this.ref.style.left = `\${left}px`; this.ref.style.width = `\${width}px`; this.ref.style.height = `\${height}px`;```

# Handling Mouse Events

The headache point in this demo is definitely to handle the mouse events manually.

We need to be careful on what behaviour should be performed when a certain type of mouse event is occurred.

For example, in the initial drawing phase, we need to utilise `mousemove` event to handle the div box resizing when dragging it.

However, we cannot directly hook the handler function to the `mousemove` event in `document.body`, because it has a pre-condition, that is we need to firstly perform `mousedown` event, which is when the user click the mouse and start to drag. Otherwise, you would see the box flickering around all the time when your mouse moves.

And you also need to remove this `mousemove` event after drawing is complete, or you would find the box keeps changing the size while your mouse moves after drawing.

So, to solve this problem, you should only add `mousemove` and `mouseup` events when `mousedown` is triggered, and remove both events when `mouseup` is actually triggered, to avoid undesired and unnecessary resize of the div box.

```function onMouseDown(e: MouseEvent) { if (!cropper) { cropper = new Cropper(document.getElementById('app')); cropper.start(e); document.body.addEventListener('mousemove', onMouseMove); document.body.addEventListener('mouseup', onMouseUp); } } function onMouseMove(e: MouseEvent) { cropper.move(e); } function onMouseUp(e: MouseEvent) { cropper.stop(e); document.body.removeEventListener('mousemove', onMouseMove); document.body.removeEventListener('mouseup', onMouseUp); } document.body.addEventListener('mousedown', onMouseDown);```

# Summary

To me, this is an interesting demo on how to create interactive elements using pure HTML, CSS and JavaScript / TypeScript.

The demo itself is not perfect. For example, it does not have dragging to reverse the div box feature. However, it is still a nice learning experience on practicing the combination of event handling and CSS drawing.

Written by Yi Zhiyue
A Software Engineer · 山不在高，有仙则灵
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