Flexbox horizontal menu centering while keeping last element to the right side [duplicate]

Answer 1

Give also #menu a left margin of auto.

Stack snippet

#container {
  display: flex;
  justify-content: center;
  align-items: center;
}
#last, #menu {
  margin-left: auto;
}

#last span, #menu span {
  padding: 10px 20px;
  background: lightgray;
}

<div id="container">
  <div id="menu">
    <span>1</span>
    <span>2</span>
    <span>3</span>
  </div>
  <div id="last">
    <span>4</span>
  </div>
</div>

---

If you want the #menu in the exact middle of its parent, we can achieve that by adding a pseudo to match the #last.

The flex: 1 will then take all the available space, and push the #menu to the middle

Stack snippet

#container {
  display: flex;
  justify-content: center;
  align-items: center;
}

#last, #container::before {
  content: '';                /*  for the pseudo to render  */
  flex: 1;                    /*  take all space left, equally  */
  text-align: right;
}

#last span, #menu span {
  padding: 10px 20px;
  background: lightgray;
}

<div id="container">
  <div id="menu">
    <span>1</span>
    <span>2</span>
    <span>3</span>
  </div>
  <div id="last">
    <span>4</span>
  </div>
</div>