microReact.ts

function el<K extends keyof HTMLElementTagNameMap>(
  tagName: K,
  props?: Partial<HTMLElementTagNameMap[K]> & {
    ref?: (r: HTMLElementTagNameMap[K]) => void;
  },
  ...children: Array<HTMLElement | string | null | undefined>
): HTMLElementTagNameMap[K] {
  const element = document.createElement(tagName);

  Object.assign(element, props);

  children.filter(Boolean).forEach((child) => {
    if (child instanceof HTMLElement) {
      element.appendChild(child);
    } else {
      const textNode = document.createTextNode(child);
      element.appendChild(textNode);
    }
  });

  if (props?.ref) {
    props.ref(element);
  }

  return element;
}

// Returns a render function. Each time render is run the old element is replaced
// with the new one in the DOM.
function DOMUpdater<T extends HTMLElement>(Component: () => T): () => T {
  let element: T | undefined;

  return () => {
    const newElement = Component();
    element?.replaceWith(newElement);
    element = newElement;
    return newElement;
  };
}

// App Component

function App() {
  let toDos: string[] = [];

  const onSubmit = (todo: string) => {
    // Add to do
    toDos.push(todo);
    render();
  };

  const onRemove = (index: number) => {
    // Remove to do
    toDos.splice(index, 1);
    render();
  };

  const render = DOMUpdater(() =>
    el(
      'div',
      { className: 'App' },
      Form({ onSubmit }),
      List({ items: toDos, onRemove }),
      toDos.length > 0 &&
        el('div', null, 'you have ', toDos.length.toString(), ' to dos')
    )
  );

  return render();
}

// Form Component

interface FormProps {
  onSubmit: (item: string) => void;
}
function Form(props: FormProps) {
  let elInput: HTMLInputElement;

  const onsubmit = (event: Event) => {
    event.preventDefault();
    props.onSubmit(elInput.value);
    elInput.value = '';
  };

  return el(
    'form',
    { onsubmit },
    el('input', { ref: (element) => (elInput = element) }),
    el('input', { type: 'submit' })
  );
}

// List Component

interface ListProps {
  items: string[];
  onRemove?: (index: number) => void;
}
function List({ items, onRemove }: ListProps) {
  const elItems = items.map((item, index) =>
    el('li', null, item, el('button', { onclick: () => onRemove(index) }, 'x'))
  );
  return el('ol', null, ...elItems);
}

document.body.appendChild(App());