src/Collection/Collection.php
public function getExtraProperty(string $key)
    {
        if (isset($this->extraProperties[$key])) {
            return $this->extraProperties[$key];
        }
    }
public function getExtraProperty(string $key)
    {
        if (isset($this->extraProperties[$key])) {
            return $this->extraProperties[$key];
        }
    }
return the value of an extra property

@return mixed