const btnToggleSideBar = document.querySelector(".btn-toggle-sidebar")
const sideBar = document.querySelector(".side-bar");
const containerBefore = document.querySelector(".before");

btnToggleSideBar.onclick = (e) => {
  containerBefore.classList.toggle("hide");
  sideBar.classList.toggle("hidden");
};

button {
  cursor: pointer;
}

.cont-outer {
  display: flex;
  position: relative;
  height: 150px;
  width: 550px;
}

.side-bar {
  background-color: red;
  transition: transform 1000ms ease;
  position: absolute;
  inset: 0 75% 0 0;
  z-index: 1;
}

.side-bar.hidden {
  transform: translateX(-100%);
}

.cont-inner {
  display: flex;
  flex-grow: 1;
}

/* this is here to just push the main content to the right so that the sidebar doesn't cover the main page*/
.before {
  flex-grow: 1;
  background-color: #a3a3a3;
  transition: flex-grow 1000ms ease;
}

/* let main page to take entire space when side bar is moved to the left */
.before.hide {
  flex-grow: 0;
}

.main-page {
  background-color: blue;
  flex-grow: 3;
}

<button class="btn-toggle-sidebar">toggle sidebar</button>
<div class="cont-outer">
  <div class="side-bar">
    <h4>list of items</h4>
    <ul>
      <li>apples</li>
      <li>oranges</li>
      <li>pineapples</li>
    </ul>
  </div>
  <div class="cont-inner">
    <div class="before"></div>
    <div class="main-page"></div>
  </div>
</div>