I tried testing this function but not full covered. I am unsure how can I test it when i use addEventListener to listen scroll event

import _ from 'lodash'
const outlineScroll = (arg: any) => {
      const currentMode = arg.currentMode
      let headingEles = getHeadings(arg) // this is array [element.offsetTop]
      let ele = arg.preview.element.style.display === 'none' ? arg[currentMode].element : arg.preview.element
      const vditorOutline = arg.outline.element
      let outline = arg.outline.element.querySelectorAll('.vditor-outline__content li span[data-target-id]')
      let top = ele.scrollTop + 35
      outline.forEach((item, i) => {
        item.className = ''
        if ( top >= headingEles[i] && top < headingEles[i+1]) {
          item.className = 'vditor-content__active'
        } else if (top >= headingEles[headingEles.length - 1]) {
          outline[headingEles.length - 1].className = 'vditor-content__active'
        }
        if (item.className.includes('vditor-content__active')) {
          if (item.offsetTop > (vditorOutline.clientHeight/10 * 9)) {
            vditorOutline.scrollTo({
              left: 0,
              top: item.offsetTop - vditorOutline.clientHeight / 2,
              behavior: "smooth"
            })
          } else if (item.offsetTop < vditorOutline.clientHeight) {
            vditorOutline.scrollTo({
              left: 0,
              top: 0,
              behavior: "smooth"
            })
          }
        }
      })
    }
let throttle = _.throttle(outlineScroll, 300, {leading: false, trailing: true})
window.addEventListener('scroll', throttle, true)

and the function argments is object just like this:

{
  currentMode: 'ir',
  preview:{
    element: '' // the value is a HTMLElement
  },
  outline:{
    element: '' // the value is a HTMLElement
  },
  ir: {
    element:  '' // the value is a HTMLElement
  }
}

so, how to expect the outline.forEach item.className and vditorOutline.scrollTo , i dont have any ideas