The answer is always three variables: your passport, your destination, and how long you are staying. Change any one and the answer changes.
Do US citizens need a visa for Japan? No: Americans visiting Japan get 90 days visa-free. Japan visa for US citizens only gets complicated for work, study, or stays over 90 days. Americans visiting India: yes, e-visa required, no exceptions. Indians visiting Dubai: pre-arranged visa required, unless they hold a valid US, UK, or EU visa, which earns a visa on arrival.
The fastest way to get the answer: enter passport plus destination into a checker and read the full result, not just "visa required" or "visa free." The day limit and the conditions are the answer. A 30-day visa-free entry and a 90-day one are different trips.
Sources: U.S. State Department, IATA Travel Centre
Do Americans need a visa for Japan? No for short tourist stays, which is why Japan tops the 'do I need a visa' questions. Visa check software answers it for any passport and destination in seconds.
Check before you book anything. Flights are refundable until they are not, and "I didn't know" has never worked at an immigration counter.