How fast is analogRead()

Yes, the MSGEQ7 would be the easiest solution, and it looks like it would be pretty easy to use with Espruino...

I'll see about adding that convolve function in Espruino 1v58 though, so you should be able to get a few bands of EQ quite easily.

I just gave FFT (using an LCD screen) a quick go:

// http://www-ee.uta.edu/eeweb/ip/Courses/DSP_new/Programs/fft.cpp
function fft(data) { // data must be 2 more than required
  var mmax, m, istep, i;
  var wtemp, wr, wpr, wpi, wi, theta;
  var tempr, tempi;
  
  var n = data.length-1;
  var j = 1;
  for (i = 1; i < n; i += 2) {
    if (j > i) {
      tempr = data[j];     data[j] = data[i];     data[i] = tempr;
      tempr = data[j+1]; data[j+1] = data[i+1]; data[i+1] = tempr;
    }
    m = n >> 1;
    while (m >= 2 && j > m) {
      j -= m;
      m >>= 1;
    }
    j += m;
  }
  mmax = 2;
  while (n > mmax) {
    istep = 2*mmax;
    theta = 2*Math.PI/mmax;
    wtemp = Math.sin(0.5*theta);
    wpr = -2.0*wtemp*wtemp;
    wpi = Math.sin(theta);
    wr = 1.0;
    wi = 0.0;
    
    for (m = 1; m < mmax; m += 2) {
      for (i = m; i <= n; i += istep) {        
        j = i + mmax;
        tempr = wr*data[j]   - wi*data[j+1];
        tempi = wr*data[j+1] + wi*data[j];
        data[j]   = data[i]   - tempr;
        data[j+1] = data[i+1] - tempi;
        data[i] += tempr;
        data[i+1] += tempi;
      }
      wr = (wtemp = wr)*wpr - wi*wpi + wr;
      wi = wi*wpr + wtemp*wpi + wi;
    }
    mmax = istep;
  }
}

SPI1.setup({ baud: 1000000, sck:B3, mosi:B5 });
var g = require("PCD8544").connect(SPI1,B6,B7,B8);

var wave = new Waveform(32);
var floatData = new Float32Array(32+1); // note the +1
wave.on("finish", function(buf) {
  floatData.set(buf,1);
  floatData[0]=0;
  fft(floatData);
  g.clear();
  for (var i=0;i<buf.length;i+=2)
    g.setPixel(i,24+Math.sqrt(floatData[i+1]*floatData[i+1] + floatData[i+2]*floatData[i+2])>>4);
  g.flip();
  // go again
  wave.startInput(A0,1000);
});

function onTimer() {
  print("Start");
 
}

setTimeout(function() {
   wave.startInput(A0,1000);
}, 1000);

It's only managing around 2 FPS at the moment. I guess with LEDs it would be significantly faster though...