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/**
 * Single file implementation of sliding windowed infinite Fourier transform (SWIFT)
 *
 * The frequency bands data is formatted like:
 * {lo: lowerBound,
 *  ctr: center,
 *  hi: higherBound}
 *
 * where lo and hi are used for calculating the necessary bandwidth for variable-Q transform spectrum visualizations and ctr for center frequency. This is generated using functions like generateFreqBands
 */
class SWIFT {
  constructor(freqBands, order = 4, timeRes = 600, sampleRate = 44100) {
    // initialize the sDFT coefficients
    this.calcCoeffs(freqBands, order, sampleRate);
    this.spectrumData = [];
  }
  
  calcCoeffs(freqBands, order = 4, timeRes = 600, sampleRate = 44100) {
    // calcCoeffs() can be called anywhere else to re-initialize sliding DFT after changes in frequency band distributions and note that x and y are used instead of real and imaginary since vector rotation is the equivalent of the complex one
    this._coeffs = [];
    freqBands.map((x, i) => {
      // rX and rY are calculated in advance here since calculating sin and cos functions are pretty slow af
      this._coeffs[i] = {
        rX: Math.cos(x.ctr*Math.PI/sampleRate),
        rY: Math.sin(x.ctr*Math.PI/sampleRate),
        decay: Math.E ** (-Math.abs(x.hi-x.lo) * 2 / sampleRate - 1/(bwOffset*sampleRate/1000)),
        coeffs: []
      };
      for (let j = 0; j < order; j++) {
        this._coeffs[i].coeffs[j] = {
          x: 0,
          y: 0
        };
      }
    });
  }
  
  analyze(dataArray) {
    const newSpectrumData = new Array(this._coeffs.length).fill(0);
    for (const x of dataArray) {
      for (let i = 0; i < this._coeffs.length; i++) {
        for (let j = 0; j < this._coeffs[i].coeffs.length; j++) {
          const input = j <= 0 ? {
            x: x,
            y: 0,
          } : this._coeffs[i].coeffs[j-1];
          this._coeffs[i].coeffs[j] = {
            x: (this._coeffs[i].coeffs[j].x * this._coeffs[i].rX - this_coeffs[i].coeffs[j].y * this._coeffs[i].rY) * (1-this._coeffs[i].decay) + input.x * this._coeffs[i].decay,
            y: (this._coeffs[i].coeffs[j].x * this._coeffs[i].rY + this_coeffs[i].coeffs[j].y * this._coeffs[i].rX) * (1-this._coeffs[i].decay) + input.y * this._coeffs[i].decay
          };
        }
        newSpectrumData[i] += this._coeffs[i].coeffs[this._coeffs.coeffs.length-1].x ** 2 +
                              this._coeffs[i].coeffs[this._coeffs.coeffs.length-1].y ** 2;
      }
    }
    spectrumData = newSpectrumData.map((x) => Math.sqrt(x));
  }
}

thanks for the code