lib/component-helpers.js
function updateSync (component, replaceNode=true) {
  if (!isValidVirtualNode(component.virtualNode)) {
    throw new Error(`${component.constructor ? component.constructor.name + ' instance' : component} is not associated with a valid virtualNode. Perhaps this component was never initialized?`)
  }

  if (component.element == null) {
    throw new Error(`${component.constructor ? component.constructor.name + ' instance' : component} is not associated with a DOM element. Perhaps this component was never initialized?`)
  }

  let newVirtualNode = component.render()
  if (!isValidVirtualNode(newVirtualNode)) {
    const namePart = component.constructor && component.constructor.name ? ' in ' + component.constructor.name : ''
    throw new Error('invalid falsy value ' + newVirtualNode + ' returned from render()' + namePart)
  }

  applyContext(component, newVirtualNode)

  syncUpdatesInProgressCounter++
  let oldVirtualNode = component.virtualNode
  let oldDomNode = component.element
  let newDomNode = patch(oldVirtualNode, newVirtualNode, {
    refs: component.refs,
    listenerContext: component
  })
  component.virtualNode = newVirtualNode
  if (newDomNode !== oldDomNode && !replaceNode) {
    throw new Error('The root node type changed on update, but the update was performed with the replaceNode option set to false')
  } else {
    component.element = newDomNode
  }

  // We can safely perform additional writes after a DOM update synchronously,
  // but any reads need to be deferred until all writes are completed to avoid
  // DOM thrashing. Requested reads occur at the end of the the current frame
  // if this method was invoked via the scheduler. Otherwise, if `updateSync`
  // was invoked outside of the scheduler, the default scheduler will defer
  // reads until the next animation frame.
  if (typeof component.writeAfterUpdate === 'function') {
    component.writeAfterUpdate()
  }
  if (typeof component.readAfterUpdate === 'function') {
    getScheduler().readDocument(function () {
      component.readAfterUpdate()
    })
  }

  syncUpdatesInProgressCounter--
}
function updateSync (component, replaceNode=true) {
  if (!isValidVirtualNode(component.virtualNode)) {
    throw new Error(`${component.constructor ? component.constructor.name + ' instance' : component} is not associated with a valid virtualNode. Perhaps this component was never initialized?`)
  }

  if (component.element == null) {
    throw new Error(`${component.constructor ? component.constructor.name + ' instance' : component} is not associated with a DOM element. Perhaps this component was never initialized?`)
  }

  let newVirtualNode = component.render()
  if (!isValidVirtualNode(newVirtualNode)) {
    const namePart = component.constructor && component.constructor.name ? ' in ' + component.constructor.name : ''
    throw new Error('invalid falsy value ' + newVirtualNode + ' returned from render()' + namePart)
  }

  applyContext(component, newVirtualNode)

  syncUpdatesInProgressCounter++
  let oldVirtualNode = component.virtualNode
  let oldDomNode = component.element
  let newDomNode = patch(oldVirtualNode, newVirtualNode, {
    refs: component.refs,
    listenerContext: component
  })
  component.virtualNode = newVirtualNode
  if (newDomNode !== oldDomNode && !replaceNode) {
    throw new Error('The root node type changed on update, but the update was performed with the replaceNode option set to false')
  } else {
    component.element = newDomNode
  }

  // We can safely perform additional writes after a DOM update synchronously,
  // but any reads need to be deferred until all writes are completed to avoid
  // DOM thrashing. Requested reads occur at the end of the the current frame
  // if this method was invoked via the scheduler. Otherwise, if `updateSync`
  // was invoked outside of the scheduler, the default scheduler will defer
  // reads until the next animation frame.
  if (typeof component.writeAfterUpdate === 'function') {
    component.writeAfterUpdate()
  }
  if (typeof component.readAfterUpdate === 'function') {
    getScheduler().readDocument(function () {
      component.readAfterUpdate()
    })
  }

  syncUpdatesInProgressCounter--
}
Synchronsly updates the DOM element associated with a component object. . This method assumes the presence of `.element` and `.virtualNode` properties on the component, which are assigned in the `initialize` function.  It calls `render` on the component to obtain the desired state of the DOM, then `diff`s it with the previous state and `patch`es the element based on the resulting diff. During the patch operation, it pushes the component's `refs` object to a shared stack so that references to DOM elements can be updated.  If `update` is called during the invocation of `updateSync`, the requests are processed synchronously as well. We track whether this is the case by incrementing and decrementing `syncUpdatesInProgressCounter` around the call.  For now, etch does not allow the root tag of the `render` method to change between invocations, because we want to preserve a one-to-one relationship between component objects and DOM elements for simplicity.