Both of these banks have exposure to China, but neither of their positions is very large relative to the capital that would absorb potential losses. Bank of America's net exposure to the East Asian country at the end of last year amounted to $10.5 billion -- see here (page 39). That's a lot, don't get me wrong, but the North Carolina-based bank has $181 billion worth of tier 1 capital.
Most of Bank of America's exposure is in the form of loans, which amount to $9.2 billion. The rest consists of open lines of credit, derivatives, and investment securities tied to the Chinese mainland.
JPMorgan Chase is in a similar position, though the composition of its exposure is different. In its third-quarter 10-Q (page 67), it reported a total of $16.3 billion in direct exposure to China. This was split roughly evenly between its trading and lending operations. Again, the total amount is a lot, but it's still a fraction of the megabank's $200 billion worth of tier 1 capital.
This isn't to say that China's troubles, and particularly as they leach into its real economy, won't impact Bank of America and JPMorgan Chase. Both banks make more money when the global economy is healthy. Demand for loans increases, as does financial activity which boosts these banks' fee-based businesses. At the same time, an improving economy translates into low loan losses, which effectively reduces expenses.
On top of this, if China's growth continues to decelerate, it's difficult to understand how that wouldn't spill over into the American economy, which is where Bank of America and JPMorgan Chase generate the majority of their revenues. If nothing else, it seems reasonable to think that fear of contagion would discourage the Federal Reserve from inching interest rates higher. This is something that banks have been chomping at the bit to happen, as higher rates will translate into higher net interest incomes, revenues, and profits.
Consequently, while there's little immediate reason for investors in Bank of America and JPMorgan Chase to be concerned about China's economic and financial travails, a downturn in the world's second largest economy is certainly not good news.