TinyChan

const TAU=Math.PI*2;
function sineWindow(x){ //x: 0 to 1
	return Math.sin(TAU*(x-.25))*.5+.5;
}
function multiply(a,b){return{r:a.r*b.r-a.i*b.i,i:a.r*b.i+a.i*b.r};}
function add(a,b){return{r:a.r+b.r,i:a.i+b.i};}
function subtract(a,b){return{r:a.r-b.r,i:a.i-b.i};}
function FFT(a,invert=false){
	let n=a.length;
	if(n===1)return;
	if(Math.log2(n)%1!==0)throw new Error("Length isn't a power of 2.");
	let a0=new Array(n/2),a1=new Array(n/2);
	for(let i=0;2*i<n;i++){a0[i]=a[2*i];a1[i]=a[2*i+1];}
	FFT(a0,invert);FFT(a1,invert);
	let ang=TAU/n*(invert?-1:1),
	    wn={r:Math.cos(ang),i:Math.sin(ang)},
	    w={r:1,i:0};
	for(let i=0;i<n/2;i++){
		let even=a0[i],odd=multiply(w,a1[i]),
		    sum=add(even,odd),difference=subtract(even,odd);
		a[i]=sum;a[i+n/2]=difference;
		if(invert){
			a[i].r/=2;a[i].i/=2;
			a[i+n/2].r/=2;a[i+n/2].i/=2;
		}
		w=multiply(w,wn);
	}
}
let mod=(n,m)=>(n%m+m)%m,
    read=(a,i)=>a[mod(Math.floor(i),a.length)],
    sine=x=>Math.sin(x*TAU),
    mix=(a,b,x)=>a+(b-a)*x;
class realtimeFFT{
  constructor(size=11){
    this.FFT_SIZE=2**size;
    this.inputBuffer=[new Float32Array(this.FFT_SIZE).fill(0),new Float32Array(this.FFT_SIZE).fill(0)];
    this.outputBuffer=[new Float32Array(this.FFT_SIZE).fill(0),new Float32Array(this.FFT_SIZE).fill(0)];
    this.bufferIndex=[0,-(2**size)*.5];
  }
  process(x,t){
    //record input
    this.inputBuffer[0][this.bufferIndex[0]]=x;
    if(this.bufferIndex[1]>=0)
      this.inputBuffer[1][this.bufferIndex[1]]=x;
    //windowing
    let win=sineWindow,
        o=this.outputBuffer[0][this.bufferIndex[0]]*win(this.bufferIndex[0]/this.FFT_SIZE)+this.outputBuffer[1][this.bufferIndex[1]]*win(this.bufferIndex[1]/this.FFT_SIZE);
    for(let bI=0;bI<2;bI++){
    this.bufferIndex[bI]++;
    if(this.bufferIndex[bI]>=this.FFT_SIZE){
      let a=[...this.inputBuffer[bI]].map(x=>({r:x,i:0}));
      FFT(a,false);
      let b=a.map(x=>({r:x.r,i:x.i}));
      for(let i=0;i<a.length;i++){
        a[i]=read(b,i*(1+sin(t*3)*.1));
      }
      FFT(a,true);
      for(let i=0;i<a.length;i++){
        this.outputBuffer[bI][i]=(a[i].r+a[i].i)*.5;//Math.sqrt(a[i].r**2+a[i].i**2);
      }
      this.bufferIndex[bI]=0;
    }
    }
    return o;
  }
}
let eff=new realtimeFFT(10);
return t=>{
  let o=(sine(t*440)+sine(t*550)+sine(t*163))*.1;
  return eff.process(o,t);
};