Virtual DOM

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./packrat-parsing/index.js ./precedence-climbing/index.py ./relational-db/index.py ./virtual-dom/index.js

§

Virtual DOM

This is an implementation of a virtual DOM, as seen in libraries like React.

It comes from Superfine (145ab11) by Jorge Bucaran:

Superfine is a minimal view layer for building web interfaces. Think Hyperapp without the framework—no state machines, effects, or subscriptions—just the absolute bare minimum (1 kB minified+gzipped). Mix it with your favorite state management library or use it standalone for maximum flexibility.

License: MIT
Copyright: © 2018 Jorge Bucaran
```
var SSR_NODE = 1,
  TEXT_NODE = 3,
  EMPTY_OBJ = {},
  EMPTY_ARR = [],
  SVG_NS = "http://www.w3.org/2000/svg"

var listener = function (event) {
  this.events[event.type](event)
}

var getKey = (vdom) => (vdom == null ? vdom : vdom.key)

var patchProperty = (node, key, oldValue, newValue, isSvg) => {
```

“key” is a not a real DOM attribute, it’s used in the diff algorithm, much like in React. See Why does React need keys? for more details.

  if (key === "key") {
  } else if (key[0] === "o" && key[1] === "n") {
    if (
      !((node.events || (node.events = {}))[(key = key.slice(2))] = newValue)
    ) {
      node.removeEventListener(key, listener)
    } else if (!oldValue) {
      node.addEventListener(key, listener)
    }
  } else if (!isSvg && key !== "list" && key !== "form" && key in node) {
    node[key] = newValue == null ? "" : newValue
  } else if (newValue == null || newValue === false) {
    node.removeAttribute(key)
  } else {
    node.setAttribute(key, newValue)
  }
}

var createNode = (vdom, isSvg) => {
  var props = vdom.props,
    node =
      vdom.type === TEXT_NODE
        ? document.createTextNode(vdom.tag)
        : (isSvg = isSvg || vdom.tag === "svg")
        ? document.createElementNS(SVG_NS, vdom.tag, { is: props.is })
        : document.createElement(vdom.tag, { is: props.is })

  for (var k in props) {
    patchProperty(node, k, null, props[k], isSvg)
  }

  for (var i = 0; i < vdom.children.length; i++) {
    node.appendChild(
      createNode((vdom.children[i] = vdomify(vdom.children[i])), isSvg)
    )
  }

  return (vdom.node = node)
}

var patchNode = (parent, node, oldVNode, newVNode, isSvg) => {
  if (oldVNode === newVNode) {
  } else if (
    oldVNode != null &&
    oldVNode.type === TEXT_NODE &&
    newVNode.type === TEXT_NODE
  ) {
    if (oldVNode.tag !== newVNode.tag) node.nodeValue = newVNode.tag
  } else if (oldVNode == null || oldVNode.tag !== newVNode.tag) {
    node = parent.insertBefore(
      createNode((newVNode = vdomify(newVNode)), isSvg),
      node
    )
    if (oldVNode != null) {
      parent.removeChild(oldVNode.node)
    }
  } else {
    var tmpVKid,
      oldVKid,
      oldKey,
      newKey,
      oldProps = oldVNode.props,
      newProps = newVNode.props,
      oldVKids = oldVNode.children,
      newVKids = newVNode.children,
      oldHead = 0,
      newHead = 0,
      oldTail = oldVKids.length - 1,
      newTail = newVKids.length - 1

    isSvg = isSvg || newVNode.tag === "svg"

§

We include oldProps here because any key that is in oldProps but not in newProps needs to be restored to its original value. See the call to node.removeAttribute in patchProperty.
```
    for (var i in { ...oldProps, ...newProps }) {
```

Optimization: skip patching if the values are the same. Some props can change through user interaction, so it’s not relevant what is in the old vnode, but rather what’s in the DOM.

      if (
        (i === "value" || i === "selected" || i === "checked"
          ? node[i]
          : oldProps[i]) !== newProps[i]
      ) {
        patchProperty(node, i, oldProps[i], newProps[i], isSvg)
      }
    }

In the loops below, we maintain two “pointers” (indices really) as we walk through the old VNode’s children and the new VNode’s children.

Old children:             New children:
[<c1>, <c2>, <c3>]        [<c1’>, <c2’>, <c3’>, <c4’>]
  ^- oldHead  ^- oldTail    ^- newHead           ^- newTail

This loops advances from the left (low indices), as long as the old and new vnode have the same key.

    while (newHead <= newTail && oldHead <= oldTail) {
      if (
        (oldKey = getKey(oldVKids[oldHead])) == null ||
        oldKey !== getKey(newVKids[newHead])
      ) {
        break
      }

      patchNode(
        node,
        oldVKids[oldHead].node,
        oldVKids[oldHead++],
        (newVKids[newHead] = vdomify(newVKids[newHead++])),
        isSvg
      )
    }

This loop advances from the right (high indices); once again, only as long as keys match.

    while (newHead <= newTail && oldHead <= oldTail) {
      if (
        (oldKey = getKey(oldVKids[oldTail])) == null ||
        oldKey !== getKey(newVKids[newTail])
      ) {
        break
      }

      patchNode(
        node,
        oldVKids[oldTail].node,
        oldVKids[oldTail--],
        (newVKids[newTail] = vdomify(newVKids[newTail--])),
        isSvg
      )
    }

§

Did we finish processing the old children?
```
    if (oldHead > oldTail) {
```
§

Create and insert nodes for the remaining new children.
```
      while (newHead <= newTail) {
```

If oldHead is an invalid index, then oldTail never moved, and new children should be appended at the end. Otherwise, insert them before oldHead.

        node.insertBefore(
          createNode((newVKids[newHead] = vdomify(newVKids[newHead++])), isSvg),
          (oldVKid = oldVKids[oldHead]) && oldVKid.node
        )
      }

§

Did we finish processing the new children?
```
    } else if (newHead > newTail) {
```

Remove the nodes for any remaining old children.

      while (oldHead <= oldTail) {
        node.removeChild(oldVKids[oldHead++].node)
      }
    } else {

§

There are both old and new children that are unprocessed.

Put all old children that have the “key” attribute into keyed.

      for (var keyed = {}, newKeyed = {}, i = oldHead; i <= oldTail; i++) {
        if ((oldKey = oldVKids[i].key) != null) {
          keyed[oldKey] = oldVKids[i]
        }
      }

      while (newHead <= newTail) {
        oldKey = getKey((oldVKid = oldVKids[oldHead]))
        newKey = getKey((newVKids[newHead] = vdomify(newVKids[newHead])))

If we’ve already seen a new vnode with a matching key…

        if (
          newKeyed[oldKey] ||
          (newKey != null && newKey === getKey(oldVKids[oldHead + 1]))
        ) {
          if (oldKey == null) {
            node.removeChild(oldVKid.node)
          }
          oldHead++
          continue
        }

        if (newKey == null || oldVNode.type === SSR_NODE) {
          if (oldKey == null) {
            patchNode(
              node,
              oldVKid && oldVKid.node,
              oldVKid,
              newVKids[newHead],
              isSvg
            )
            newHead++
          }
          oldHead++
        } else {
          if (oldKey === newKey) {
            patchNode(node, oldVKid.node, oldVKid, newVKids[newHead], isSvg)
            newKeyed[newKey] = true
            oldHead++
          } else {
            if ((tmpVKid = keyed[newKey]) != null) {
              patchNode(
                node,
                node.insertBefore(tmpVKid.node, oldVKid && oldVKid.node),
                tmpVKid,
                newVKids[newHead],
                isSvg
              )
              newKeyed[newKey] = true
            } else {
              patchNode(
                node,
                oldVKid && oldVKid.node,
                null,
                newVKids[newHead],
                isSvg
              )
            }
          }
          newHead++
        }
      }

      while (oldHead <= oldTail) {
        if (getKey((oldVKid = oldVKids[oldHead++])) == null) {
          node.removeChild(oldVKid.node)
        }
      }

Remove nodes for any keys that have disappeared.

      for (var i in keyed) {
        if (newKeyed[i] == null) {
          node.removeChild(keyed[i].node)
        }
      }
    }
  }

  return (newVNode.node = node)
}

var vdomify = (newVNode) =>
  newVNode !== true && newVNode !== false && newVNode ? newVNode : text("")

var recycleNode = (node) =>
  node.nodeType === TEXT_NODE
    ? text(node.nodeValue, node)
    : createVNode(
        node.nodeName.toLowerCase(),
        EMPTY_OBJ,
        EMPTY_ARR.map.call(node.childNodes, recycleNode),
        SSR_NODE,
        node
      )

var createVNode = (tag, props, children, type, node) => ({
  tag,
  props,
  key: props.key,
  children,
  type,
  node,
})

export var text = (value, node) =>
  createVNode(value, EMPTY_OBJ, EMPTY_ARR, TEXT_NODE, node)

export var h = (tag, props, children = EMPTY_ARR) =>
  createVNode(tag, props, Array.isArray(children) ? children : [children])

export var patch = (node, vdom) => (
  ((node = patchNode(
    node.parentNode,
    node,
    node.vdom || recycleNode(node),
    vdom
  )).vdom = vdom),
  node
)