<nav class="menu js-menu">
  <div class="menu__contents js-menu-contents">
    <button class="menu__toggle js-menu-toggle">
      <h1 class="menu__title js-menu-title">Menu</h1>
    </button>
    <ul class="menu__items">
      <li class="menu__item">Menu item 1</li>
      <li class="menu__item">Menu item 2</li>
      <li class="menu__item">Menu item 3</li>
      <li class="menu__item">Menu item 4</li>
    </ul>
  </div>
</nav>

html, body {
  background: #666;
  font-family: Arial, Helvetica, sans-serif;
}

.menu {
  position: absolute;
  opacity: 0;
  pointer-events: none;
  transform-origin: top left;
  overflow: hidden;
  border-radius: 3px;
  box-shadow: 0 3px 3px rgba(0, 0, 0, 0.4);
  background: #FFF;
  will-change: transform;
}

.menu__contents {
  transform-origin: top left;
  will-change: transform;
}

.menu__toggle {
  text-align: left;
  padding: 0;
  margin: 0;
  border: none;
  background: none;
  cursor: pointer;
  width: 100%;
}

.menu__title {
  padding: 0;
  margin: 0;
  font-weight: 500;
  display: inline-block;
  padding: 8px 12px;
}

.menu__items {
  position: relative;
  list-style: none;
  padding: 0;
  margin: 0;
  background: #FFF;
  z-index: 1;
}

.menu__item {
  padding: 8px 12px;
  border-top: 1px solid #DDD;
}

.menu--active {
  opacity: 1;
  pointer-events: auto;
}

.menu--expanded {
  animation-name: menuExpandAnimation;
  animation-duration: 0.2s;
  animation-timing-function: linear;
}

.menu__contents--expanded {
  animation-name: menuExpandContentsAnimation;
  animation-duration: 0.2s;
  animation-timing-function: linear;
}

.menu--collapsed {
  animation-name: menuCollapseAnimation;
  animation-duration: 0.2s;
  animation-timing-function: linear;
}

.menu__contents--collapsed {
  animation-name: menuCollapseContentsAnimation;
  animation-duration: 0.2s;
  animation-timing-function: linear;
}

'use strict';

class Menu {
  constructor () {
    this._menu = document.querySelector('.js-menu');
    this._menuContents = this._menu.querySelector('.js-menu-contents');
    this._menuToggleButton = this._menu.querySelector('.js-menu-toggle');
    this._menuTitle = this._menu.querySelector('.js-menu-title');

    this._expanded = true;
    this._animate = false;
    this._collapsed;

    this.expand = this.expand.bind(this);
    this.collapse = this.collapse.bind(this);
    this.toggle = this.toggle.bind(this);

    this._calculateScales();
    this._createEaseAnimations();
    this._addEventListeners();

    this.collapse();
    this.activate();
  }

  activate () {
    this._menu.classList.add('menu--active');
    this._animate = true;
  }

  collapse () {
    if (!this._expanded) {
      return;
    }
    this._expanded = false;

    const {x, y} = this._collapsed;
    const invX = 1 / x;
    const invY = 1 / y;

    this._menu.style.transform = `scale(${x}, ${y})`;
    this._menuContents.style.transform = `scale(${invX}, ${invY})`;
    
    if (!this._animate) {
      return;
    }

    this._applyAnimation({expand: false});
  }

  expand () {
    if (this._expanded) {
      return;
    }
    this._expanded = true;

    this._menu.style.transform = `scale(1, 1)`;
    this._menuContents.style.transform = `scale(1, 1)`;
      
    if (!this._animate) {
      return;
    }

    this._applyAnimation({expand: true});
  }

  toggle () {
    if (this._expanded) {
      this.collapse();
      return;
    }

    this.expand();
  }

  _addEventListeners () {
    this._menuToggleButton.addEventListener('click', this.toggle);
  }

  _applyAnimation ({expand}=opts) {
    this._menu.classList.remove('menu--expanded');
    this._menu.classList.remove('menu--collapsed');
    this._menuContents.classList.remove('menu__contents--expanded');
    this._menuContents.classList.remove('menu__contents--collapsed');

    // Force a recalc styles here so the classes take hold.
    window.getComputedStyle(this._menu).transform;

    if (expand) {
      this._menu.classList.add('menu--expanded');
      this._menuContents.classList.add('menu__contents--expanded');
      return;
    }

    this._menu.classList.add('menu--collapsed');
    this._menuContents.classList.add('menu__contents--collapsed');
  }

  _calculateScales () {
    const collapsed = this._menuTitle.getBoundingClientRect();
    const expanded = this._menu.getBoundingClientRect();

    this._collapsed = {
      x: collapsed.width / expanded.width,
      y: collapsed.height / expanded.height
    }
  }

  _createEaseAnimations () {
    let menuEase = document.querySelector('.menu-ease');
    if (menuEase) {
      return menuEase;
    }

    menuEase = document.createElement('style');
    menuEase.classList.add('menu-ease');

    const menuExpandAnimation = [];
    const menuExpandContentsAnimation = [];
    const menuCollapseAnimation = [];
    const menuCollapseContentsAnimation = [];
    for (let i = 0; i <= 100; i++) {
      const step = this._ease(i/100);
      const startX = this._collapsed.x;
      const startY = this._collapsed.y;
      const endX = 1;
      const endY = 1;

      // Expand animation.
      this._append({
        i,
        step,
        startX: this._collapsed.x,
        startY: this._collapsed.y,
        endX: 1,
        endY: 1,
        outerAnimation: menuExpandAnimation,
        innerAnimation: menuExpandContentsAnimation
      });

      // Collapse animation.
      this._append({
        i,
        step,
        startX: 1,
        startY: 1,
        endX: this._collapsed.x,
        endY: this._collapsed.y,
        outerAnimation: menuCollapseAnimation,
        innerAnimation: menuCollapseContentsAnimation
      });
    }

    menuEase.textContent = `
      @keyframes menuExpandAnimation {
        ${menuExpandAnimation.join('')}
      }

      @keyframes menuExpandContentsAnimation {
        ${menuExpandContentsAnimation.join('')}
      }
      
      @keyframes menuCollapseAnimation {
        ${menuCollapseAnimation.join('')}
      }

      @keyframes menuCollapseContentsAnimation {
        ${menuCollapseContentsAnimation.join('')}
      }`;

    document.head.appendChild(menuEase);
    return menuEase;
  }

  _append ({
        i,
        step,
        startX, 
        startY, 
        endX, 
        endY, 
        outerAnimation, 
        innerAnimation}=opts) {

    const xScale = startX + (endX - startX) * step;
    const yScale = startY + (endY - startY) * step;

    const invScaleX = 1 / xScale;
    const invScaleY = 1 / yScale;

    outerAnimation.push(`
      ${i}% {
        transform: scale(${xScale}, ${yScale});
      }`);

    innerAnimation.push(`
      ${i}% {
        transform: scale(${invScaleX}, ${invScaleY});
      }`);
  }

  _clamp (value, min, max) {
    return Math.max(min, Math.min(max, value));
  }

  _ease (v, pow=4) {
    v = this._clamp(v, 0, 1);

    return 1 - Math.pow(1 - v, pow);
  }
}

new Menu()