A simple preact prop checker that uses custom functions to verify props.

preact-prop-checker.js

import preact from 'preact';

function printWarning(message) {
  message = `Warning: ${message}`;
  if (typeof console !== 'undefined') {
    console.error(message);
  }
}

// installs global prop checker
if (process.env.NODE_ENV !== 'production') {
  preact.options.vnode = vnode => {
    const propTypes = vnode.nodeName.propTypes;
    if (propTypes) {
      for (const propName in propTypes) {
        const propValue = vnode.attributes[propName];
        const {
          checker,
          message=`Invalid prop on ${propValue}`,
          isRequired=false
        } = propTypes[propName];

        if (propValue === undefined && isRequired) {
          printWarning(`${propName} is required`);
        } else if (!checker(propValue)) {
          printWarning(message);
        }
      }
    }
  }
}