Is a credit card M1 or M2?

I'm curious, when we're discussing the classification of money supply, would a credit card be considered part of M1 or M2? M1, as I understand, includes the most liquid forms of money such as cash and checking account balances. While M2 is a broader category that encompasses M1 plus savings deposits, small-denomination time deposits, and retail money market mutual fund shares. How does a credit card, which essentially allows for borrowing against a line of credit, fit into this classification? Is it more akin to the readily available funds of M1, or does it fall under the broader umbrella of M2 due to its connection to savings and investment vehicles?