JavaScript mouse drawing on the canvas 👨‍🎨

Today I wanted to continue canvas explorations by checking out how to draw on the canvas with our mouse.

It turns out it's actually fairly simple and easy to implement!

We'll be building this cool drawing app. (Have a play around!)

HTML Structure

The HTML could not be simpler, all we need is one big canvas.

<canvas id="canvas"></canvas>

Styling our app

As for our styling, all we need to do is remove our default margin, create a cool emoji cursor, and set the width/height to be the same size as the viewport.

* {
  margin: 0;
  padding: 0;
cursor:url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg'  width='40' height='48' viewport='0 0 100 100' style='fill:black;font-size:24px;'><text y='50%'>✍️</text></svg>") 16 0,auto;
}
canvas {
  width: 100vw;
  height: 100vh;
}

Drawing on canvas with JavaScript mouse

Now on to the fun part, the JavaScript to connect our mouse movements to the canvas.

Let's start by defining our variables.

const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");
let coord = { x: 0, y: 0 };

We need the canvas, and retrieve it based on it's ID.
Then we get the context (where we actually draw on)

Then we define our base coordinates.

Now let's attach listeners for:

• mousedown (start registering our drawing
• mouseup (stop the drawing)
• resize (resize the canvas)

document.addEventListener("mousedown", start);
document.addEventListener("mouseup", stop);
window.addEventListener("resize", resize);

Let's start with the resize function, this function will resize the canvas based on our viewport. It will make the canvas 100% or the width and height.

We also call this function right away.

function resize() {
  ctx.canvas.width = window.innerWidth;
  ctx.canvas.height = window.innerHeight;
}

resize();

Let's define our mousedown (start) function.

function start(event) {
  document.addEventListener("mousemove", draw);
  reposition(event);
}

This function will invoke the listener for mousemove, so we don't have to keep listening to it.
Then we call our reposition function, which will register our mouse position.

The reposition function will look like this:

function reposition(event) {
  coord.x = event.clientX - canvas.offsetLeft;
  coord.y = event.clientY - canvas.offsetTop;
}

On to the stop function.

function stop() {
  document.removeEventListener("mousemove", draw);
}

We only need to stop listening to our register mousemove function.

The last function we will make is the draw. This actually will create lines on the canvas.

function draw(event) {
  ctx.beginPath();
  ctx.lineWidth = 5;
  ctx.lineCap = "round";
  ctx.strokeStyle = "#ACD3ED";
  ctx.moveTo(coord.x, coord.y);
  reposition(event);
  ctx.lineTo(coord.x, coord.y);
  ctx.stroke();
}

In order:

• We begin a new path.
• We set the line width to 5 pixels.
• We change the line endings to round.
• We set the color to blue.
• We change our position to the initial position and move the canvas point to the moved position.
• Then we draw a line between these two points.
• Last we call the stroke to colour it.

That's it. We can now draw lines on the canvas.

If you want to read more about canvas, check out these articles.

• Getting started with the HTML canvas
• Vanilla JavaScript save canvas as an image
• Vanilla JavaScript colouring our canvas elements 🌈
• Vanilla JavaScript images in canvas
• Vanilla JavaScript canvas images to black and white
• Checkboxify your images with JavaScript ✅

Thank you for reading, and let's connect!

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Comments

[rzeczuchy]

That's a really cool sample :)

I know that this is beyond the scope of this short article, but one thing that is worth looking into for anyone wanting to build a drawing app like this is Bresenham's line algorithm.

Drawing lines with paths is OK as long as you are using a round brush (lineCap = "round"), but if you use a large square brush, you're going to see significant tearing when drawing an irregular line. Bresenham's algo lets you avoid that problem - it's a great and cheap way to approximate lines.

[Kevin Hoyt]

Just wanted to second on using a line algorithm. If you draw with the mouse quickly, you will also see that the event does not fire with every pixel, so you get gaps in the data points. A line algorithm solves that, and can perform better depending on the algorithm and number of data points. Also consider pointer events or detecting touch for mobile support.

[Chris Bongers]

Yes touch was indeed on the list still, but probably be a separate one, I like to keep the articles short so people will actually try them.
The line algorithm is a good one, I need to do some research on that.

[Chris Bongers]

Oh nice addition, good read!

[max]

Pretty neat. I used to teach a similar exercice.

I'd recommand using lowdash.debounce or throttle to optimize events (limit the drawing, as JS basic event system is consuming hard ^^')

[Chris Bongers]

Good advice, but I wanted to showcase it in plain javascript, without using any packages.
Generally tracking mousemove fulltime is pretty hard yes, for some reason, this doesn't seem to spike CPU much.

[Arpit]

wow

[Chris Bongers]

Thank you Arpit