鼠标左右拖动改变左右盒子大小功能。

<div id='box' className='box'>
   <div id='left' className='left'>左边盒子</div>
   <div id='resize' className='resize' />
   <div id='right' className='right'>右边盒子</div>
</div>

  .box {
    display: flex;
    width: 100%;
    height: 300px;
    background: #fff;
    .left {
      width: 300px;
      text-align: center;
    }
    .right {
      flex: 1;
      text-align: center;
    }
    .resize{
      cursor: w-resize;
      position: relative;
      border: 1px solid #cecece;
    }
  }

  window.onload = function () {
    let resize = document.getElementById("resize");
    let left = document.getElementById("left");
    let right = document.getElementById("right");
    let box = document.getElementById("box");
    resize.onmousedown = function (e) {
      let startX = e.clientX;
      resize.left = resize.offsetLeft;
      document.onmousemove = function (e) {
        let endX = e.clientX;
        let moveLen = resize.left + (endX - startX);
        let maxT = box.clientWidth - resize.offsetWidth;
        if (moveLen > maxT - 500) moveLen = maxT - 500;
        resize.style.left = moveLen;
        left.style.width = moveLen + "px";
        right.style.width = (box.clientWidth - moveLen - 5) + "px";
      }
      document.onmouseup = function (evt) {
        evt.stopPropagation()
        document.onmousemove = null;
        document.onmouseup = null;
        resize.releaseCapture && resize.releaseCapture();
      }
      resize.setCapture && resize.setCapture();
      return false;
    }
  }