Is there any kind of mutual funds which can never have a loss, like maybe a currency fund?
Money market funds are mutual funds that invest in very safe cash, cash equivalents, and short-term debt. Although it is possible for a money market fund to lose value (“breaking the buck”), it is very rare for these funds to do so.
Of course, the trade off is that money market funds also have an extremely low rate of return. For example, the Vanguard Prime Money Market Fund (VMMXX) currently has a 10-year average annual return of 0.63%.
If you are looking for an investment that is guaranteed by the government never to lose money, you’ll need to be in an FDIC-insured bank account, with even lower rates of return.