CleanCanvas.js

// Lower value increases trail duration.
const CANVAS_CLEANUP_ALPHA = 0.3;

// Cleans up the canvas by removing older trails.
// ...
function cleanCanvas() {
    // Set 'destination-out' composite mode, so additional fill doesn't remove non-overlapping content.
    context.globalCompositeOperation = 'destination-out';
    // Set alpha level of content to remove.
    // Lower value means trails remain on screen longer.
    context.fillStyle = `rgba(0, 0, 0, ${CANVAS_CLEANUP_ALPHA})`;
    // Fill entire canvas.
    context.fillRect(0, 0, canvas.width, canvas.height);
    // Reset composite mode to 'lighter', so overlapping particles brighten each other.
    context.globalCompositeOperation = 'lighter';
}