render.js

function render(element, parentDom) {
  const { type, props } = element;

  // Create DOM element
  const isTextElement = type === "TEXT ELEMENT";
  const dom = isTextElement
    ? document.createTextNode("")
    : document.createElement(type);

  // Add event listeners
  const isListener = name => name.startsWith("on");
  Object.keys(props).filter(isListener).forEach(name => {
    const eventType = name.toLowerCase().substring(2);
    dom.addEventListener(eventType, props[name]);
  });

  // Set properties
  const isAttribute = name => !isListener(name) && name != "children";
  Object.keys(props).filter(isAttribute).forEach(name => {
    dom[name] = props[name];
  });

  // Render children
  const childElements = props.children || [];
  childElements.forEach(childElement => render(childElement, dom));

  // Append to parent
  parentDom.appendChild(dom);
}