<div class="wrap">
        <div class="content"></div>
        <div class="scroll">
            <div class="bar"></div>
        </div>
    </div>

* {
            margin: 0;
            padding: 0;
            user-select: none;
        }
        
        .wrap {
            overflow: hidden;
            position: relative;
            width: 400px;
            height: 600px;
            background-color: #f1f1f1;
            margin: 10px auto;
            box-shadow: 0 0 10px red;
        }
        
        .content {
            position: absolute;
            left: 0;
            top: 0;
            width: 380px;
            padding: 10px;
            line-height: 30px;
            text-indent: 2em;
        }
        
        .scroll {
            position: absolute;
            top: 0;
            right: 0;
            width: 16px;
            height: 600px;
            background-color: #ccc;
        }
        
        .bar {
            position: absolute;
            width: 16px;
            border-radius: 10px;
            background-color: #333;
        }

var oWrap = document.querySelector('.wrap');
        var oContent = document.querySelector('.content');
        var oScroll = document.querySelector('.scroll');
        var oBar = document.querySelector('.bar');
        var rate = 0;
        //初始函数
        initSCroll();

        function initSCroll() {
            //初始bar的高度；
            oBar.style.height = oWrap.offsetHeight / oContent.offsetHeight * oScroll.offsetHeight + 'px';
            //内容展示比例
            rate = (oContent.offsetHeight - oWrap.offsetHeight) / (oScroll.offsetHeight - oBar.offsetHeight);

        };
        var eventTypeMap = {
            isDown: false,
            startPos: {
                y: 0,
                top: 0
            },
            'mousedown': function(e) {
                this.isDown = true;
                this.startPos.y = e.clientY;
                this.startPos.top = oBar.offsetTop;
            },
            'mousemove': function(e) {
                if (!this.isDown) {
                    return false;
                }
                var _y = e.clientY - this.startPos.y + this.startPos.top;
                scrollBar(_y);
            },
            'mouseup': function(e) {
                if (!this.isDown) {
                    return false;
                }
                this.isDown = false;
            },
            'mousewheel': function(e) {
                var _y = oBar.offsetTop;
                if (e.wheelDelta > 0) {
                    _y -= 10;
                } else {
                    _y += 10;
                }
                scrollBar(_y);
            }

        };
        //事件
        oBar.addEventListener('mousedown', eventSort, false);
        document.addEventListener('mousemove', eventSort, false);
        document.addEventListener('mouseup', eventSort, false);
        oBar.addEventListener('mousewheel', eventSort, false);
        //事件分流
        function eventSort(e) {
            if (eventTypeMap[e.type] && typeof eventTypeMap[e.type] === 'function') {
                eventTypeMap[e.type](e);
            }
        };

        function scrollBar(_y) {
            _y = Math.max(0, _y);
            _y = Math.min(_y, oScroll.offsetHeight - oBar.offsetHeight);
            oBar.style.top = _y + 'px';
            oContent.style.marginTop = -_y * rate + 'px';
        }