Immediate mode DOM diffing (a.k.a. incremental DOM). Applicable to native platforms (WinForms, Android)

example.js

import { patch, begin, end, text } from "./immediate.js";

var count = 0;

function tick(event) {
  if (event.type === "click") {
    if (event.target.id === "increment") {
      count++;
    } else if (event.target.id === "decrement") {
      count--;
    }
  }
  patch(document.body, function () {
    begin("p");
    {
      text("Count: ");
      text(count);
    }
    end();
    begin("p");
    {
      begin("button", { id: "increment" });
      {
        text("Increment");
      }
      end();
      begin("button", { id: "decrement" });
      {
        text("Decrement");
      }
      end();
    }
    end();
    begin("ul");
    for (var i = 0; i < count; i++) {
      begin("li");
      {
        text("Item ");
        text(i);
      }
      end();
    }
    end();
  });
}

addEventListener("DOMContentLoaded", tick);
addEventListener("click", tick);

immediate.js

var parent;
var stack;

export function patch(container, render) {
  parent = container;
  stack = [container.firstChild];
  render();
  clearRemainder();
}

export function begin(tag, attrs) {
  patchTag(tag, attrs);
  parent = getCurr();
  stack.push(parent.firstChild);
}

export function end() {
  clearRemainder();
  parent = parent.parentNode;
  stack.pop();
  moveNext();
}

export function text(value) {
  var curr = getCurr();
  if (!curr) {
    var node = document.createTextNode(value);
    parent.appendChild(node);
    setCurr(node);
  } else if (curr.nodeType !== 3) {
    var node = document.createTextNode(value);
    parent.replaceChild(node, curr);
    setCurr(node);
  } else if (curr.data !== String(value)) {
    curr.data = String(value);
  }
  moveNext();
}

export function tag(tag, attrs) {
  patchTag(tag, attrs);
  moveNext();
}

function getCurr() {
  return stack[stack.length - 1];
}

function setCurr(curr) {
  stack[stack.length - 1] = curr;
}

function moveNext() {
  stack[stack.length - 1] = getCurr().nextSibling;
}

function createElement(tag, attrs) {
  var el = document.createElement(tag);
  for (var name in attrs) {
    el.setAttribute(name, attrs[name]);
  }
  return el;
}

function clearRemainder() {
  var curr = getCurr();
  while (curr) {
    var next = curr.nextSibling;
    parent.removeChild(curr);
    curr = next;
  }
}

function patchTag(tag, attrs) {
  var curr = getCurr();
  if (!curr) {
    var el = createElement(tag, attrs);
    parent.appendChild(el);
    setCurr(el);
  } else if (curr.localName !== tag) {
    var el = createElement(tag, attrs);
    parent.replaceChild(el, getCurr());
    setCurr(el);
  } else {
    for (var name in attrs) {
      var value = String(attrs[name]);
      if (curr.getAttribute(name) !== value) {
        curr.setAttribute(name, value);
      }
    }
    for (var i = curr.attributes.length - 1; i > -1; i--) {
      var name = curr.attributes[i].name;
      if (!(name in attrs)) {
        curr.removeAttribute(name);
      }
    }
  }
}