Edit in JSFiddle

(() => {

	const form = document.querySelector('form[name=bmi]');
  
  const onSubmit = event => {
  	
    event.preventDefault();
    
  };
  
  form.addEventListener('submit', onSubmit, false);
	
})();
<form name="bmi">
  <h1>BMI Calculator</h1>
  <label>
    <input type="text" name="weight" placeholder="Weight (kg)">
  </label>
  <label>
    <input type="text" name="height" placeholder="Height (cm)">
  </label>
  <button type="submit">
    Calculate BMI
  </button>
  <div class="calculation">
    <div>
      BMI calculation: <span class="result"></span>
    </div>
    <div>
      This means you are: <span class="health"></span>
    </div>
  </div>
</form>