one euro filter Typescript version

one-euro-filter.ts

/* eslint-disable max-classes-per-file */
class LowPassFilter {
  y: number | null;

  s: number | null;

  alpha = 0;

  constructor(alpha: number) {
    this.setAlpha(alpha);
    this.y = null;
    this.s = null;
  }

  setAlpha(alpha: number) {
    if (alpha <= 0 || alpha > 1.0) {
      throw new Error();
    }
    this.alpha = alpha;
  }

  filter(value: number, timestamp: number, alpha: number) {
    if (alpha) {
      this.setAlpha(alpha);
    }
    let s;
    if (!this.y) {
      s = value;
    } else {
      s = this.alpha * value + (1.0 - this.alpha) * this.s!;
    }
    this.y = value;
    this.s = s;
    return s;
  }

  lastValue() {
    return this.y;
  }
}

export default class OneEuroFilter {
  freq: number;

  minCutOff: number;

  beta: number;

  dCutOff: number;

  x: LowPassFilter;

  dx: LowPassFilter;

  lasttime: number | null;

  constructor(freq: number, minCutOff = 1.0, beta = 0.0, dCutOff = 1.0) {
    if (freq <= 0 || minCutOff <= 0 || dCutOff <= 0) {
      throw new Error();
    }
    this.freq = freq;
    this.minCutOff = minCutOff;
    this.beta = beta;
    this.dCutOff = dCutOff;
    this.x = new LowPassFilter(this.alpha(this.minCutOff));
    this.dx = new LowPassFilter(this.alpha(this.dCutOff));
    this.lasttime = null;
  }

  alpha(cutOff: number) {
    const te = 1.0 / this.freq;
    const tau = 1.0 / (2 * Math.PI * cutOff);
    return 1.0 / (1.0 + tau / te);
  }

  filter(x: number, timestamp: number | null = null) {
    if (this.lasttime && timestamp) {
      this.freq = 1.0 / (timestamp - this.lasttime);
    }
    this.lasttime = timestamp;
    const prevX = this.x.lastValue();
    const dx = !prevX ? 0.0 : (x - prevX) * this.freq;
    const edx = this.dx.filter(dx, timestamp!, this.alpha(this.dCutOff));
    const cutOff = this.minCutOff + this.beta * Math.abs(edx);
    return this.x.filter(x, timestamp!, this.alpha(cutOff));
  }
}

credits go to : https://gist.github.com/3846masa/5628f711e86fd62bea56b18e32177c60
I only converted to Typescript