Nayuki: in-place single-precision complex-complex. Around 7kb.
Nayuki (obj): Nayuki with the sin/cos tables pre-calculated on object construction. Around 4kb.
Nockert: double-precision real-complex. Around 25kb.
Dntj: double-precision complex-complex. Forward transform is scaled and I've scaled it back again here, which may introduce rounding error. Around 10kb.
Cross: double-precision real-complex in C, compiled with Emscripten. This is considered a slow implementation amongst native code ones. Around 60kb.
KissFFT: single-precision real-complex in C, compiled with Emscripten. A reasonably sophisticated implementation. Around 70kb.
FFTW: single-precision real-complex in C, compiled with Emscripten. GPL licensed. Around 3Mb.