Sandisk bested Strategy and surprised some investors in being included in the S&P 500 index.

Sandisk Corp. will join the S&P 500 on Friday, S&P Dow Jones Indices said late Monday.

Shares of Sandisk $(SNDK)$, which makes computer-storage devices, rallied more than 7% in the extended session after the news. The company is moving to the S&P 500 SPX from the S&P Small Cap 600 SML, replacing advertising and marketing company Interpublic Group of Companies Inc. $(IPG)$.

The changes in the index are effective at the open on Friday, a Thanksgiving holiday-abbreviated session for U.S. equity markets.

Sandisk, with a market capitalization of about $34 billion as of Monday, had stuck out in the small-cap index for being much bigger in terms of market value than the others.

Sandisk also bested Strategy Inc. (MSTR), an enterprise-software company that has called itself the largest corporate holder of cryptocurrency. Strategy, formerly known as MicroStrategy, had been newly eligible for the S&P 500 inclusion.

Monday's news came a bit earlier than some investors had anticipated.

Expectations were brewing that the S&P changes to include Sandisk or Strategy would have come in early December, as part of a regularly scheduled rebalance.

The index provider has discretion, however, in choosing when and how to make any changes or even to forgo changes, as it happened in June.

Sandisk's stock has gained nearly 400% in the past three months, riding the artificial-intelligence boom.

A place in the S&P 500 is a coveted spot for companies, as it exposes their shares to a much broader range of investors, including passive funds that track the benchmark and actively managed funds that may have restrictions on where they can invest.