Unit testing a randomly generated string

I have the following method:

generateKey: () => {
    let key =
      Math.random()
        .toString(36)
        .substring(2, 15) +
      Math.random()
        .toString(36)
        .substring(2, 15);

    return key;
  }

This method generates a random character string and I wish to unit test it.

After searching how to unit test randomness I've come across the following answers but not a concrete application:

• Answer 01
• Answer 02

How may I test randomness in this specific case?

Testing the length of the string and its content is simple enough, but testing that values were truly generated randomly seems impossible.