React custom validator with isRequired support

chainablePropType.js

const chainablePropType = predicate => {
  const propType = (props, propName, componentName) => {
    // don't do any validation if empty
    if (props[propName] == null) {
      return;
    }

    return predicate(props, propName, componentName);
  }

  propType.isRequired = (props, propName, componentName) => {
    // warn if empty
    if (props[propName] == null) {
      return new Error(`Required prop \`${propName}\` was not specified in \`${componentName}\`.`);
    }

    return predicate(props, propName, componentName);
  }

  return propType;
}

const customProp = chainablePropType(() => {...});
const propTypes = {
  prop: customProp,
  requiredProp: customProp.isRequired
};