body composition calculations.js

class BodyMetrics {
  constructor(params) {
    Object.keys(params).forEach(key => {
      this[key] = params[key];
    });
    this.results = {};
    this.bodyFatPercentage();
    this.bodyFatWeight();
    this.bodyFatDensity();
  }

  bodyFatDensity() {
    var age = +this.age;
    var gender = this.gender;
    var chest = +this.measurements.calipers.chest;
    var abdomen = +this.measurements.calipers.abdomen;
    var thigh = +this.measurements.calipers.thigh;
    var waist = +this.measurements.girth.waist;
    var forearm = +this.measurements.girth.forearm;
    var triceps = +this.measurements.calipers.triceps;
    var suprailiac = +this.measurements.calipers.suprailiac;
    var glute = +this.measurements.girth.glute;
    var canCalcBFD = (gender === 'M' && (age && thigh && chest && abdomen && waist && forearm)) || (triceps && thigh && suprailiac && age && glute);
    if (!canCalcBFD) {
      this.results.bodyFatDensity = null;
      return;
    }
    if (gender === 'M') {
      var sum = chest + abdomen + thigh;
      var sum_sq = sum * sum;
      this.results.bodyFatDensity = 1.0990750 - (0.0008209 * sum) + (0.0000026 * sum_sq) - (0.0002017 * age) - (0.005675 * (waist / 100)) + (0.018586 * (forearm / 100));
    } else {
      var sum = triceps + thigh + suprailiac;
      var sum_sq = sum * sum;
      this.results.bodyFatDensity = 1.1470292 - (0.0009376 * sum) + (0.0000030 * sum_sq) - (0.0001156 * age) - (0.0005839 * glute);
    }
  }

  bodyFatWeight() {
    if (!this.results.BodyFatPercentage || !this.weight) {
      this.results.BodyFatWeight = null;
      this.results.LeanBodyWeight = null;
      return;
    }
    this.results.BodyFatWeight = (this.weight * this.results.BodyFatPercentage) / 100;
    this.results.LeanBodyWeight = this.weight - this.results.BodyFatWeight;
  }

  bodyFatPercentage() {
    var gender = this.gender;
    var age = +this.age;
    var triceps = +this.measurements.calipers.triceps;
    var abdomen = +this.measurements.calipers.abdomen;
    var thigh = +this.measurements.calipers.thigh;
    var suprailiac = +this.measurements.calipers.suprailiac;
    var canCalcBFP = gender && age && triceps && abdomen && thigh && suprailiac;
    if (!canCalcBFP) {
      this.results.BodyFatPercentage = null;
      this.results.BodyFatWeight = null;
      this.results.LeanBodyWeight = null;
      this.results.BodyFatDensity = null;
      return;
    }
    var sum = triceps + suprailiac + abdomen + thigh;
    var sum_sq = sum * sum;
    var bfp = gender === 'M' ?
      (0.29288 * sum) - (0.0005 * sum_sq) + (0.15845 * age) - 5.76377 :
      (0.29669 * sum) - (0.00043 * sum_sq) + (0.02963 * age) + 1.4072;
    this.results.BodyFatPercentage = bfp;
  }
}

// formulas from: https://www.topendsports.com/testing/density-jackson-pollock.htm

var bm = new BodyMetrics({
  age: 31, // years
  gender: 'M',
  weight: 240, // lbs
  measurements: {
    calipers: { // millimeters
      chest: 30,
      triceps: 20,
      abdomen: 45,
      thigh: 30,
      suprailiac: 5
    },
    girth: { // centimeters
      waist: 100,
      forearm: 25
    }
  }
});

console.log(bm.results);