Scroll the browser window up and down at a configurable pixels per second.

scrollDownAtRate.js

/**
 * Smoothly scrolls the entire page body from top to bottom
 * at a specified rate in pixels per second.
 *
 * @param {number} ratePxPerSecond - The scroll speed in pixels per second (e.g., 100).
 */
function scrollDownAtRate(ratePxPerSecond) {
    // Check for a valid rate
    if (ratePxPerSecond <= 0) {
        console.error("Scroll rate must be a positive number.");
        return;
    }

    // Determine the maximum scroll position (the bottom of the page)
    const targetScrollPosition = document.documentElement.scrollHeight - window.innerHeight;

    // Check if the page is already at the bottom or if it's not scrollable
    if (window.scrollY >= targetScrollPosition) {
        console.log("Already at the bottom of the page.");
        return;
    }

    let prevTimestamp = null;

    /**
     * The animation loop function, called by requestAnimationFrame.
     * @param {number} timestamp - The time in milliseconds passed since the page loaded.
     */
    function animateScroll(timestamp) {
        if (!prevTimestamp) {
            prevTimestamp = timestamp;
        }

        // Calculate the time elapsed (delta time) since the last frame in milliseconds
        const deltaTime = timestamp - prevTimestamp;

        // Calculate the distance to scroll in this frame:
        // Distance = Rate (px/s) * Time (s)
        const distanceThisFrame = ratePxPerSecond * (deltaTime / 1000);

        // Update the previous timestamp for the next frame calculation
        prevTimestamp = timestamp;

        // Check if the calculated scroll will overshoot the target
        const currentScroll = window.scrollY;
        const remainingScroll = targetScrollPosition - currentScroll;

        if (remainingScroll <= distanceThisFrame) {
            // Scroll the remaining distance precisely to the bottom
            window.scrollBy(0, remainingScroll);
            console.log("Scroll to bottom complete.");
            return; // Stop the animation loop
        } else {
            // Scroll the calculated distance
            window.scrollBy(0, distanceThisFrame);
        }

        // Continue the animation loop
        window.requestAnimationFrame(animateScroll);
    }

    // Start the animation loop
    window.requestAnimationFrame(animateScroll);
}

// --- Example Usage ---
// Scroll the page at a rate of 100 pixels per second.
// The total time taken will depend on the height of the page.

// scrollDownAtRate(100);