Spotlight effect with JS and CSS

Spotlight effects can be a pretty creative way of revealing content on your website. And they're surprisingly easy to create with a little JS and CSS!

The basic idea

TLDR; Use CSS radial gradient backgrounds, and then use JS to track the mouse movement and move the center of the gradient.

There are other, more complex ways of doing this - you could use canvas or external libraries - but I find the simpler approach is a good starting point and should work well enough for most use cases.

Let's get started:

#spotlight {
  position: fixed;
  opacity: 1;
  width: 100vw;
  height: 100vh;
  top: 0;
  left: 0;
  pointer-events: none;
}

<div id="spotlight"></div>

const spotlightEl = document.querySelector("#spotlight");

function handleMouseMove(event) {
    const { clientX, clientY } = event;
    
    spotlightEl.style.background = `radial-gradient(circle at ${clientX}px ${clientY}px, #00000000 10px, #000000ee 350px)`;
}

document.addEventListener("mousemove", handleMouseMove)

And...that's pretty much it.

The #spotlight div is positioned so that it always covers the entire viewport. We then attach an event listener to listen to the mouse's movement, and set the X and Y directly in the radial-gradient CSS value.

The #00000000 10px, #000000ee 350px part basically means we want a gradient that is transparent at it's center, and at 350px from the center is a slightly transparent black. That's how the gradient center reveals what's beneath.

Making it configurable

So far we have a basic spotlight, so let's go ahead and improve it.

We can create a Spotlight class that allows you to easily set up new spotlights and pass in some options:

• toggleEl is the id of the element you want to use as a trigger to toggle the spotlight.
• innerRadius and outerRadius set the spotlight size.
• outerColor sets the background color (try passing in red for a truly bleeding edge experience).

The class also provides methods for switching the spolight on and off.

Making the light feel more natural

To make it a look a little more natural, there are a couple of added touches.

First, there is a slight delay in the spotlight movement. This adds a sense of weight to the light, like we're dragging it around with our mouse.

handleMouseMove(event: MouseEvent) {
    setTimeout(() => {
      this.updateEl(event.clientX, event.clientY);
    }, 50);
}

Also, you'll notice the light "pulses" slightly - continually increasing and decreasing.

But how do we get it to work?

My first instinct was to animate the gradient size, but that's not actually possible in CSS! Next I thought of somehow changing the outerRadius value in steps inside a function called in a setInterval, but that turned out to be a really stupid idea. Finally, I came up with the simple solution - animate the scale of the entire spotlight div with a CSS animation.

@keyframes pulse {
  0% {
    transform: scale(1);
  }
  100% {
    transform: scale(1.1);
  }
}

this.el.style.animation =
        "pulse 3s ease-in-out infinite alternate forwards";

This way, we don't need to touch the actual gradient, we just animate the entire "canvas", stretching the gradient in the process. I also used CSS animations for the "switching on/off" part.

Wrapping up

Obviously, you shouldn't be using spotlights in every single page (please don't!). But when used right, they're a nice way for users to feel like they're discovering some cool concealed content for the first time.

Thanks for reading!