app.html

<template>
  <nav class="sidebar ${isOpen ? 'open' : 'closed'}">
    <button click.delegate="toggle()">
      hamburger
    </button>
  </nav>
  <main class="main"></main>
</template>

app.scss

.sidebar {
  position: absolute;
  top: 0;
  left: 0;
  bottom: 0;
  transition: width ease-in-out 250ms;
  
  & .open {
    width: 260px;
  }
  
  & .closed {
    width: 50px;
  }
}

app.ts

export class AppViewModel {

  isOpen: boolean;

  toggle() {
    this.isOpen = !this.isOpen;
  }

}