health-calculator/src/body_measurement.ts

A library to help calculate well known health-related metrics.

import {
	Gender,
	inchToCm,
	lbToKg,
} from './util';

/**
 * @module body_measurement
 */

enum BodyCategory {
	ExtremelySlim = 'Extremely Slim',
	Slim = 'Slim',
	Healthy = 'Healthy',
	Overweight = 'Overweight',
	VeryOverweight = 'Very Overweight',
	Obese = 'Obese',
}

/**
 * Calculates body mass index (BMI).
 *
 * @param {number} weight - Weight of a person.
 * @param {number} height - Height of a person.
 * @param {boolean} imperial - Flag to use imperial (lb and inch). Default to use metric (kg and cm).
 * @returns {number} BMI.
 */
export function bmi( weight: number, height: number, imperial?: boolean ): number {
	// TODO: validate weight and height.
	if ( imperial ) {
		weight = lbToKg( weight );
		height = inchToCm( height );
	}
	return Number( ( weight / ( height / 100 ) ** 2 ).toFixed( 2 ) );
}

/**
 * Calculates waist-to-height ratio (WHtR).
 *
 * @see {@link https://en.wikipedia.org/wiki/Waist-to-height_ratio}
 * @param {number} waistCircumference - Waist circumference of a person.
 * @param {number} height - Height of a person.
 * @param {boolean} imperial - Flag to use imperial (lb and inch). Default to use metric (kg and cm).
 * @returns {number} WHtR.
 */
export function whtr( waistCircumference: number, height: number, imperial?: boolean ) : number {
	if ( imperial ) {
		waistCircumference = inchToCm( waistCircumference );
		height = inchToCm( height );
	}
	return Number( ( waistCircumference / height ).toFixed( 2 ) );
}

/**
 * Categorize the boundaries for WHtR in terms of their health implications.
 *
 * @see {@link https://en.wikipedia.org/wiki/Waist-to-height_ratio#Health_implications}
 * @param {Gender} gender - 'male' or 'female'.
 * @param {number} age - Age between 18 and 120.
 * @param {number} ratio - WHtR.
 * @returns {BodyCategory} Category.
 */
export function whtrCategory( gender: Gender, age: number, ratio: number ) : BodyCategory {
	if ( age < 1 || age > 120 ) {
		throw new Error( 'age must be between 1 and 120' );
	}

	if ( age <= 15 ) {
		return whtrChildren( ratio );
	}

	if ( gender === Gender.Male ) {
		return whtrMale( ratio );
	}

	return whtrFemale( ratio );
}

/**
 * WHtR category for children (up to 15 yeard old).
 *
 * @param {number} ratio - WHtR.
 * @returns {BodyCategory} Category.
 */
function whtrChildren( ratio: number ) : BodyCategory {
	if ( ratio <= 0.34 ) {
		return BodyCategory.ExtremelySlim;
	}
	if ( ratio >= 0.35 && ratio <= 0.45 ) {
		return BodyCategory.Slim;
	}
	if ( ratio >= 0.46 && ratio <= 0.51 ) {
		return BodyCategory.Healthy;
	}
	if ( ratio >= 0.52 && ratio <= 0.63 ) {
		return BodyCategory.Overweight;
	}
	return BodyCategory.Obese;
}

/**
 * WHtR category for adult male.
 *
 * @param {number} ratio - WHtR.
 * @returns {BodyCategory} Category.
 */
function whtrMale( ratio: number ) : BodyCategory {
	if ( ratio <= 0.34 ) {
		return BodyCategory.ExtremelySlim;
	}
	if ( ratio >= 0.35 && ratio <= 0.42 ) {
		return BodyCategory.Slim;
	}
	if ( ratio >= 0.43 && ratio <= 0.52 ) {
		return BodyCategory.Healthy;
	}
	if ( ratio >= 0.53 && ratio <= 0.57 ) {
		return BodyCategory.Overweight;
	}
	if ( ratio >= 0.58 && ratio <= 0.62 ) {
		return BodyCategory.VeryOverweight;
	}
	return BodyCategory.Obese;
}

/**
 * WHtR category for adult female.
 *
 * @param {number} ratio - WHtR.
 * @returns {BodyCategory} Category.
 */
function whtrFemale( ratio: number ) : BodyCategory {
	if ( ratio <= 0.34 ) {
		return BodyCategory.ExtremelySlim;
	}
	if ( ratio >= 0.35 && ratio <= 0.41 ) {
		return BodyCategory.Slim;
	}
	if ( ratio >= 0.42 && ratio <= 0.48 ) {
		return BodyCategory.Healthy;
	}
	if ( ratio >= 0.49 && ratio <= 0.53 ) {
		return BodyCategory.Overweight;
	}
	if ( ratio >= 0.54 && ratio <= 0.57 ) {
		return BodyCategory.VeryOverweight;
	}
	return BodyCategory.Obese;
}

/**
 * Calculates waist-to-hip ratio (WHR).
 *
 * @see {@link https://en.wikipedia.org/wiki/Waist%E2%80%93hip_ratio}
 * @param {number} waistCircumference - Waist circumference of a person.
 * @param {number} hipCircumference - Hip circumference of a person.
 * @param {boolean} imperial - Flag to use imperial (lb and inch). Default to use metric (kg and cm).
 * @returns {number} WHR.
 */
export function whr( waistCircumference: number, hipCircumference: number, imperial?: boolean ) : number {
	if ( imperial ) {
		waistCircumference = inchToCm( waistCircumference );
		hipCircumference = inchToCm( hipCircumference );
	}
	return Number( ( waistCircumference / hipCircumference ).toFixed( 2 ) );
}

/**
 * Calculates body fat percentage (BFP) using BMI.
 *
 * @see {@link https://en.wikipedia.org/wiki/Body_fat_percentage#From_BMI}
 * @param {Gender} gender - 'male' or 'female'.
 * @param {number} age - Age between 18 and 120.
 * @param {number} bmi - BMI result.
 * @returns {number} BFP.
 */
export function bfp( gender: Gender, age: number, bmi: number ) : number {
	// TODO: validate age and maybe bmi.
	if ( gender === Gender.Male ) {
		return Number( ( ( 1.20 * bmi ) + ( 0.23 * age ) - 16.2 ).toFixed( 2 ) );
	}
	return Number( ( ( 1.20 * bmi ) + ( 0.23 * age ) - 5.4 ).toFixed( 2 ) );
}

/**
 * Calculates lean body mass (LBM) using the Boer formula.
 *
 * @see {@link https://en.wikipedia.org/wiki/Lean_body_mass#Boer[3]}
 * @param {Gender} gender - 'male' or 'female'.
 * @param {number} weight - Weight of a person.
 * @param {number} height - Height of a person.
 * @param {boolean} imperial - Flag to use imperial (lb and inch). Default to use metric (kg and cm).
 * @returns {number} LBM.
 */
export function lbm( gender: Gender, weight: number, height: number, imperial?: boolean ) : number {
	if ( imperial ) {
		weight = lbToKg( weight );
		height = inchToCm( height );
	}

	if ( gender === Gender.Male ) {
		return Number( ( ( 0.407 * weight ) + ( 0.267 * height ) - 19.2 ).toFixed( 2 ) );
	}

	return Number( ( ( 0.252 * weight ) + ( 0.473 * height ) - 48.3 ).toFixed( 2 ) );
}

/**
 * Calculates relative fat mass (RFM).
 *
 * @param {Gender} gender - 'male' or 'female'.
 * @param {number} height - Height of a person.
 * @param {number} waistCircumference - Waist circumference of a person.
 * @param {boolean} imperial - Flag to use imperial (lb and inch). Default to use metric (kg and cm).
 * @returns {number} RFM.
 */
export function rfm( gender: Gender, height: number, waistCircumference: number, imperial?: number ) : number {
	if ( imperial ) {
		height = inchToCm( height );
		waistCircumference = inchToCm( waistCircumference );
	}

	if ( height < 80 || height > 272 ) {
		throw new Error( 'height must be between 80cm and 272cm' );
	}

	if ( gender === Gender.Male ) {
		return Number( ( 64 - ( 20 * height / waistCircumference ) ).toFixed( 0 ) );
	}
	return Number( ( 76 - ( 20 * height / waistCircumference ) ).toFixed( 0 ) );
}

/**
 * Calculates fat-free mass index (FFMI).
 *
 * @param {number} weight - Weight of a person.
 * @param {number} height - Height of a person.
 * @param {number} bodyFat - Body fat percentage.
 * @param {boolean} imperial - Flag to use imperial (lb and inch). Default to use metric (kg and cm).
 * @returns {number} FFMI.
 */
export function ffmi( weight: number, height: number, bodyFat: number, imperial?: number ) : number {
	if ( imperial ) {
		weight = lbToKg( weight );
		height = inchToCm( height );
	}

	const ffm = weight * ( 1 - ( bodyFat / 100 ) );
	return Number( ( ffm / ( ( height / 100 ) ** 2 ) ).toFixed( 2 ) );
}