index.js

// converting pounds to kg
function poundsToKg(pounds) {
  kilograms = pounds / 2.205;
  return kilograms;
}

// converting inches to meters
function inchesToM(inches) {
  centimeters = (inches * 2.54) / 100;
  return centimeters;
}

function calculateBMI() {
  // defining variables corresponding to the needed DOM elements
  const radioPounds = document.getElementById("radioPounds");
  const radioKgs = document.getElementById("radioKgs");
  const radioCms = document.getElementById("radioCms");
  const radioInches = document.getElementById("radioInches");
  const weightInput = Number(document.getElementById("weightInput").value);
  const heightInput = Number(document.getElementById("heightInput").value);
  const result = document.getElementById("result");

  // declaring the bmi value before calculation
  let bmi;
  // making sure the user selects units
  if (
    (!radioPounds.checked && !radioKgs.checked) ||
    (!radioCms.checked && !radioInches.checked)
  ) {
    result.textContent = "Please select units!";
  } else if (heightInput <= 0 || weightInput <= 0) {
    result.textContent = "Enter valid numbers!"; // making sure the user puts valid values (greater than 0)
  } else {
    if (radioKgs.checked) {
      // kg + cm
      if (radioCms.checked) {
        bmi = weightInput / (heightInput / 100) ** 2;
      }
      // kg + pounds
      else {
        bmi = weightInput / inchesToM(heightInput) ** 2;
      }
    } else if (radioPounds.checked) {
      // pounds + cm
      if (radioCms.checked) {
        bmi = poundsToKg(weightInput) / (heightInput / 100) ** 2;
      }
      // pounds + inches
      else {
        bmi = poundsToKg(weightInput) / inchesToM(heightInput) ** 2;
      }
    }

    // showing the result in different colors based on bmi value
    let index;
    let colors = ["#c0a330", "#3db120", "#e07426", "#e31f18"];
    let comments = ["underweight", "normal weight", "overweight", "obesity"];
    if (bmi < 18.5) {
      index = 0;
    } else if (bmi <= 24.9) {
      index = 1;
    } else if (bmi <= 29.9) {
      index = 2;
    } else {
      index = 3;
    }

    // range information for different bmi values is taken from https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html

    result.textContent = `BMI: ${bmi.toFixed(1)}. Weight status: ${
      comments[index]
    }.`;
    result.style.color = colors[index];
  }
}