Sample data

const data = new ComplexArray(1024);
data.map((value, i, n) => {
  value.real = (i > n/3 && i < 2*n/3) ? 1 : 0;
});
      

Transform (in place):
data.FFT();

Simple low pass filter:

data.map((freq, i, n) => {
  if (i > n/5 && i < 4*n/5) {
    freq.real = 0;
    freq.imag = 0;
  }
});
      

Transform back:
data.InvFFT();

... or all in one step

data.frequencyMap((freq, i, n) => {
  if (i > n/5 && i < 4*n/5) {
    freq.real = 0;
    freq.imag = 0;
  }
});