SO Developer Storyのような垂直タイムラインを作成する質問する

// wrap in anonymous function to not pollute global namespace
(function() {

  // find the minimium element in an array, and the index of it
  function min(arr) {
    var ind = -1, min = false, i;
    for (i = 0; i < arr.length; ++i) {
      if (min === false || arr[i] < min) {
        ind = i;
        min = arr[i];
      }
    }
    return {
      index: ind,
      value: min
    };
  }

  // position events within a single timeline container
  function updateTimeline(timeline, minSpace = 10) {
    var events = Array.from(timeline.querySelectorAll('.event')),
        tops = [0, 0],
        bottoms = [0, 0],
        minTops = [0, 0];
    // determine height of container, upper bound
    timeline.style.height = events.reduce(function(sum, event) {
      return sum + event.offsetHeight;
    }, 0) + 'px';

    // position events in timeline
    events.forEach(function(event) {
      // find highest point to put event at
      // first put it with its top aligned to the lowest bottom - this will
      // always yield a good solution, we check better ones later
      var h = event.offsetHeight,
          y = min(bottoms),
          x = (y.index === 0 ? 0 : 50),
          b = y.value + h,
          m = (tops[1 - y.index] + bottoms[1 - y.index]) / 2;
      event.className = 'event';
      event.classList.add('event--' + (y.index === 0 ? 'left' : 'right'));
      // try to squeeze element up as high as possible
      // first try to put midpoint of new event just below the midpoint of
      // the last placed event on the other side
      if (m + minSpace - h / 2 > minTops[y.index]) {
        y.value = m + minSpace - h / 2;
        b = y.value + h;
      }
      // it would be even better if the top of the new event could move
      // all the way up - this can be done if its midpoint is below the
      // midpoint of the last event on the other side
      if (minTops[y.index] + h / 2 > m + minSpace) {
        y.value = minTops[y.index];
        b = y.value + h;
      }

      // update tops and bottoms for current event
      tops[y.index] = Math.ceil(y.value);
      bottoms[y.index] = Math.ceil(b + minSpace);
      minTops[y.index] = bottoms[y.index];

      // update tops and bottoms for other side, as applicable
      if (y.value + (h / 2) + minSpace > bottoms[1 - y.index]) {
        tops[1 - y.index] = bottoms[1 - y.index];
        minTops[1 - y.index] = bottoms[1 - y.index];
      }
      bottoms[1 - y.index] = Math.ceil(Math.max(
        bottoms[1 - y.index],
        y.value + (h / 2) + minSpace
      ));

      // put event at correct position
      event.style.top = y.value + 'px';
      event.style.left = x + '%';
    });

    // set actual height of container
    timeline.style.height = (Math.max.apply(null, bottoms) - minSpace) + 'px';
  }

  // position events within all timeline containers on the page
  function updateAllTimelines(minSpace = 10) {
    Array.from(document.querySelectorAll('.timeline'))
      .forEach(function(timeline) {
        updateTimeline(timeline, minSpace);
      });
  }

  // initialize timeline by calling above functions
  var space = 10;
  updateAllTimelines(space);
  window.addEventListener('resize', function() {
    updateAllTimelines(space);
  });

}());

/* line in center */
.timeline {
  position: relative;
  width: 100%;
}

.timeline:before {
  content: " ";
  display: block;
  position: absolute;
  top: 0;
  left: calc(50% - 2px);
  width: 4px;
  height: 100%;
  background: red;
}

/* events */
.event {
  position: absolute;
  top: 0;
  left: 0;
  box-sizing: border-box;
  width: 50%;
  height: auto;
  padding: 0 20px 0 0;
}
.event--right {
  /* Move padding to other side.
   * It is important that the padding does not change, because
   * changing the height of an element (which padding can do)
   * while layouting is not handled by the JavaScript. */
  padding: 0 0 0 20px;
}

/* discs on timeline */
.event:after {
  content: "";
  display: block;
  position: absolute;
  top: calc(50% - 5px);
  width: 0;
  height: 0;
  border-style: solid;
}
.event--left:after {
  right: 0;
  border-width: 10px 0 10px 20px;
  border-color: transparent transparent transparent #de2d26;
}
.event--right:after {
  left: 0;
  border-width: 10px 20px 10px 0;
  border-color: transparent #de2d26 transparent transparent;
}

/* event styling */
.event__body {
  padding: 20px;
  background: #de2d26;
  color: rgba(255, 255, 255, 0.9);
}

<div class="timeline">
  <div class="event">
    <div class="event__body">
      Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus varius sodales purus, id gravida ipsum accumsan quis. Donec ultrices orci quis ex consequat mollis. Etiam in gravida enim. Etiam efficitur lorem id turpis auctor gravida. Ut condimentum dolor nibh, in hendrerit leo egestas at.
    </div>
  </div>
  <div class="event">
    <div class="event__body">
      Nam magna felis, malesuada vitae elementum sit amet, tempus sed eros. Etiam ullamcorper elementum viverra. Morbi dictum metus id nibh congue, at lacinia felis iaculis. Etiam pretium augue in erat lobortis viverra. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Suspendisse facilisis, velit vel placerat faucibus, massa est eleifend turpis, ac tempor dui turpis at mi. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos.
    </div>
  </div>
  <div class="event">
    <div class="event__body">
      Mauris malesuada arcu sed lacus tristique, a eleifend sem egestas. Pellentesque at malesuada nisi.
    </div>
  </div>
  <div class="event">
    <div class="event__body">
      Nam et purus at elit vulputate molestie ac non eros. Proin mattis ligula velit, ut semper eros venenatis a. Vestibulum sagittis consectetur diam, molestie dapibus sem viverra a. Fusce felis augue, mollis id massa molestie, sagittis rutrum risus. In vel molestie elit, eget fringilla ante. Sed et elit blandit, tincidunt leo non, vehicula mi. Aenean tempus tincidunt eros vel tincidunt. Integer orci orci, gravida sit amet elit nec, luctus condimentum est.
    </div>
  </div>
  <div class="event">
    <div class="event__body">
      Pellentesque sodales ultrices sem, eget convallis ante condimentum id. Fusce ac turpis ac ex tincidunt malesuada a ac est.
    </div>
  </div>
</div>