JSFiddle - React, Tailwind, and code Playground

JavaScript

class TempScrollBox {
  constructor() {
    this.scrollBarWidth = 0;
    
    this.measureScrollbarWidth();
  }

  measureScrollbarWidth() {
    // Add temporary box to wrapper
    let scrollbox = document.createElement('div');

    // Make box scrollable
    scrollbox.style.overflow = 'scroll';

    // Append box to document
    document.body.appendChild(scrollbox);

    // Measure inner width of box
    this.scrollBarWidth = scrollbox.offsetWidth - scrollbox.clientWidth;

    // Remove box
    document.body.removeChild(scrollbox);
  }

  get width() {
    return this.scrollBarWidth;
  }
}

let scrollbox = new TempScrollBox();

// Alert number (for testing)
alert(`The width of your scrollbar is ${scrollbox.width}px`);