Code for blogpost "Using JSX for your own lightweight declarative library": https://medium.com/@mikaelbrevik/using-jsx-for-your-own-lightweight-declarative-library-a773d3796475

own-jsx-15.js

const isFunction = s => typeof s === "function";

function createElement(tag, props, ...children) {
  // If first argument is function, we know it is custom component,
  // So call it as a function, passing the props with children as a property.
  // Using spread like this will work when props is `null` also
  if (isFunction(tag)) {
    return tag({ ...props, children });
  }
  
  // Same as before
  const el = attrs(document.createElement(tag), props);
  children.forEach(child => {
    const node = !isString(child) ? child : document.createTextNode(child);
    el.appendChild(node);
  });
  
  return el;
}