自定义滚动条

js

class Scroller {
    constructor() {
        this.parent = document.querySelector('.parent')
        this.child = document.querySelector('.child')
        this.scroller = document.querySelector('.scroller')
        this.slider = document.querySelector('.slider')

        this.listen()
        this.setSliderWidth()
    }

    listen = () => {
        this.parent.onscroll = this.onScroll
        this.scroller.onclick = this.onClickScroller
        this.slider.onmousedown = this.onDrag
    }

    setSliderWidth = () => {
        let percent = 100
        const maxWidth = this.parent.clientWidth
        let width = this.child.scrollWidth
        percent = Math.min(maxWidth / width * 100, 100)
        const newWidth = `${percent.toFixed(3)}%`

        const sliderStyle = this.slider.style
        if (newWidth !== sliderStyle.width) {
            sliderStyle.width = newWidth
        }
    }

    onScroll = e => {
        const maxWidth = this.scroller.clientWidth
        const sliderWidth = this.slider.clientWidth
        const { scrollLeft, scrollWidth, clientWidth } = this.parent

        const percent = scrollLeft / (scrollWidth - clientWidth)
        this.slider.style.left = `${(maxWidth - sliderWidth) * percent}px`
    }

    onClickScroller = e => {
        if (this.startX) return
        const { left, width } = this.scroller.getBoundingClientRect()
        const scrollWidth = this.parent.scrollWidth
        const percent = (e.pageX - left) / width
        this.parent.scrollLeft = percent * scrollWidth
    }

    onDrag = (e) => {
        this.startX = e.pageX - this.slider.offsetLeft
        document.addEventListener('mousemove', this.onMouseMove)
        document.addEventListener('mouseup', this.onMouseUp)
    }

    onMouseMove = (e) => {
        const maxWidth = this.scroller.clientWidth
        const sliderWidth = this.slider.clientWidth
        const left = e.pageX - this.startX
        const body = this.parent
        const rate = left / (maxWidth - sliderWidth)
        body.scrollLeft = rate * (body.scrollWidth - body.clientWidth)
    }

    onMouseUp = (e) => {
        setTimeout(() => this.startX = null)
        document.removeEventListener('mousemove', this.onMouseMove)
        document.removeEventListener('mouseup', this.onMouseUp)
    }
    
}
new Scroller()

dom

<div class="box">
    <div class="parent">
        <div class="child">0123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899</div>
    </div>
    <div class="scroller">
        <div class="slider"></div>
    </div>
</div>

style

div::-webkit-scrollbar {
    display: none;
}
.box {
    position: fixed;
    left: 0; right: 0; top: 0; bottom: 0; margin: auto;
     200px; 
    height: 200px;
}
.parent {
     100%; height: 100%;
    background: #333;
    overflow: scroll;
    border-radius: 4px;
}
.child {
    margin-top: 10px;
    background: #ddd;
     max-content;
}
.scroller {
     100%;
    height: 8px;
    background: #e4e4e4;
    position: absolute;
    bottom: 0;
    border-radius: 4px;
    cursor: default;
    transition: all .4s;
}
.slider {
    height: 100%;
    background: #c5c5c5;
    border-radius: 4px;
    position: relative;
    cursor: default;
}