code: function validateValueByRule(accumulator, value, key, property, rule) {
    // If value is not an object, we can't validate it by the rule.
    // A previous validator will have reported this error.
    if (!isObject(value)) {
      return accumulator;
    }
  
    // property does not exist on value, this is valid
    if (!Object.prototype.hasOwnProperty.call(value, property)) {
      return accumulator;
    }
  
    const propertyValue = value[property];
    if (propertyValue === undefined) {
      accumulator.errors.push(new Error(`property "${property}" is undefined for key "${key}"`));
      return accumulator;
    }
  
    switch (rule.type) {
      case 'equal':
        if (
          !isString(propertyValue)
          && !isNumber(propertyValue)
          && !isBoolean(propertyValue)
        ) {
          accumulator.errors.push(new Error(`equal property "${property}" must be a string, number, or boolean for key "${key}"`));
        }
        break;
      case 'list':
        if (!Array.isArray(propertyValue)) {
          accumulator.errors.push(new Error(`list property "${property}" must be an array for key "${key}"`));
          return accumulator;
        }
        propertyValue.forEach((listValue) => {
          if (
            !isString(listValue)
            && !isNumber(listValue)
            && !isBoolean(listValue)
          ) {
            accumulator.errors.push(new Error(`values in list property "${property}" must be a string, number, or boolean for key "${key}"`));
          }
        });
        break;
      case 'percent':
        if (
          !isNumber(propertyValue)
          || propertyValue > 100
          || propertyValue < 0
        ) {
          accumulator.errors.push(new Error(`percent property "${property}" must be a number between 0 and 100 inclusive for key "${key}"`));
        } else {
          accumulator.percentTotal += propertyValue;
        }
        break;
      /* eslint default-case: 0 */
    }
  
    return accumulator;
  }
docs: This is used in a reduce handler.  Accumulator should have two properties, errors (array) and percentTotal (number)