In the example examples | p5.js of the Frequency Spectrum you can show the spectrum of the sound which is currently played.
Is it possible to get the whole spectrum of an file using FFT analyze from p5 sound without being played?
no it has to play but you can connect the sound only to the analyzer by setInput (in theory; I haven’t tested yet). I found the implementation of p5.sound a bit confusing because they all default to output to master - maybe you need to explicitly disconnect from the master.