HeartIcon.jsx

const HeartIcon = (props = {}) => (
  <svg {...props} viewBox='0 0 ${width} ${height}'>
    <g><path d='M38.723 12c-7.187 0-11.16 7.306-11.723 8.131C26.437 19.306 22.504 12 15.277 12 8.791 12 3.533 18.163 3.533 24.647 3.533 39.964 21.891 55.907 27 56c5.109-.093 23.467-16.036 23.467-31.353C50.467 18.163 45.209 12 38.723 12z'></path></g>
  </svg>
);

Where are width and height defined? I would expect to see props.width and props.height in the template string.

It's done with ES6 spread operator {...props}