I want to be able to find arbitrary freq values.

fft.js

var nda = require('ndarray')
var fft = require('ndarray-fft')
var mag = require('ndarray-complex').mag
var sr = 48000
var range = sr / 2

var ab = new Float32Array(sr)
var out = new Float32Array(sr)

for(var x = 0; x < sr; x++){

  var signal = Math.sin((x / sr) * Math.PI * 2 * 441) * 4
  signal +=  Math.sin((x / sr) * Math.PI * 2 * 33) / 4
  ab[x] = signal
}

ab = nda(ab, [sr, 1])
out = nda(out, [sr, 1])

fft(1, ab, out)
mag(ab, ab, out)
var pd, pfreq

for(var i = 0; i < ab.data.length; i++){
  var freq = i * sr / ab.data.length
  var d = ab.data[i]
  if(freq === 441) console.log(d) // getting 36000000 
  if(freq === 33) console.log(d) // getting 9216001024
}