html代码：
<div class="bar">
        <div class="line">
            <div class="dot"></div>
        </div>
</div>

//js:
// 获取进度条
        var bar = document.querySelector(".bar");
        var line = document.querySelector(".line");
        var dot = document.querySelector(".dot");

        dot.addEventListener('mousedown', function () {
            document.addEventListener('mousemove', move);
            function move(e){
                // 获取鼠标到bar左边的距离
                var x = e.pageX - bar.offsetLeft;
                // 当鼠标位置小于进度条最小位置
                if ( e.pageX < bar.offsetLeft) {
                    line.style.width = 0 + "px";
                    dot.style.left = 0 + "px";
                }
                // 大于
                else if( e.pageX > bar.offsetLeft + bar.offsetWidth){
                    line.style.width = bar.width + "px";
                    dot.style.left = bar.width + "px";
                }else{
                    line.style.width = x + "px";
                    dot.style.left = x + "px";
                }
            }
            document.addEventListener('mouseup', function () {
                // 移除由addEventListener添加的方法
                document.removeEventListener('mousemove',move);
            })
        })