JavaScript – Set Scrollbar Size as a CSS Variable

2 January 2024

—

Managing layout shifts and space allocation in web design can be tricky when considering the variable size of scrollbars across different browsers and operating systems. This JavaScript function, dynamically calculates the size of the scrollbar and sets it as a CSS custom property, making it easier to handle layout adjustments.

Function Overview

const putScrollbarSizeInCSSVariable = () => {
  let timeout = false
  const delay = 250

  window.addEventListener('resize', () => {
    clearTimeout(timeout)
    timeout = setTimeout(() => {
      document.documentElement.style.setProperty('--scrollbarsize', `${window.innerWidth - document.documentElement.clientWidth}px`)
    }, delay)
  })

  setTimeout(() => {
    document.documentElement.style.setProperty('--scrollbarsize', `${window.innerWidth - document.documentElement.clientWidth}px`)
  }, 0)
}

The function putScrollbarSizeInCSSVariable performs the following actions:

It initializes a resize event listener on the window object.
When a resize event occurs, the function debounces the event handling using a timeout to avoid excessive calculations and reflows.
After a specified delay, the actual width of the scrollbar is calculated by subtracting the clientWidth of the documentElement (which does not include the scrollbar) from the innerWidth of the window (which does).
The calculated scrollbar size is then set as a CSS custom property --scrollbarsize on the documentElement (typically the <html> element).

Debouncing Mechanism

The function uses a debouncing mechanism to ensure that the scrollbar size is recalculated at most every 250 milliseconds during continuous resizing, rather than on every frame, which helps to improve performance.

Immediate Invocation

The function also sets the scrollbar size immediately upon invocation, ensuring that the variable is available as soon as the script runs.

Use Case

This function can be particularly useful in responsive design scenarios where the presence of a scrollbar might affect the overall layout, such as in full-width carousel sliders or modal dialogs.

Integration

To use this function, simply call putScrollbarSizeInCSSVariable() in your script. This sets up everything automatically and maintains the --scrollbarsize CSS variable with the correct scrollbar size, even as the user resizes their browser window.

CSS Access

In your CSS, access the variable with var(--scrollbarsize) to adjust layout or compensate for the scrollbar width when necessary.

Conclusion

Incorporating putScrollbarSizeInCSSVariable into a web project provides a dynamic and responsive solution for managing layouts that take the scrollbar into account, ensuring a more consistent and visually stable user experience.