digest(encoding) {
	    if (this.string !== undefined) {
	      this.hash.update(this.string, this.encoding);
	    }

	    return this.hash.digest(encoding);
	  }
Calculates the digest {@link https://nodejs.org/api/crypto.html#crypto_hash_digest_encoding}
@param {string=} encoding encoding of the return value
@returns {string|Buffer} digest