Javascript：获取Typeerror-元素为null

Question

I have a BMI calculator in Wordpress, that get's the

Typeerror:

result is null and health is null

The calculator works fine in fiddle https://jsfiddle.net/2au9y34x/2/ but don't on Wordpress. I have placed the script in the footer, so that the HTML is read first, so that is not where the problem is. Can anyone help solve this problem?

Javascript is:

<script type='text/javascript'>
  var form = document.querySelector('form[name=bmi]');
  var onSubmit = function(event) {

    event.preventDefault();
    var healthMessage;

    var result = form.querySelector('.result');
    var health = form.querySelector('.health');

    var weight = parseInt(form.querySelector('input[name=weight]').value, 10);
    var height = parseInt(form.querySelector('input[name=height]').value, 10);

    var bmi = (weight / (height / 100 * height / 100)).toFixed(1);

    if (bmi < 18.5) {
      healthMessage = 'undervægtig';
    } else if (bmi > 18.5 && bmi < 25) {
      healthMessage = 'normal vægtig';
    } else if (bmi > 25) {
      healthMessage = 'overvægtig';
    }
    result.innerHTML = bmi;
    health.innerHTML = healthMessage;
  }
  form.addEventListener('submit', onSubmit, false);
</script>

HTML is:

<form name="bmi">
  <h1>Mål dit BMI:</h1>
  <label>
    <input type="text" name="weight" id="weight" placeholder="Vægt (kg)">
    <input type="text" name="height" id="height" placeholder="Højde (cm)">
    <input type="submit" name="submit" id="submit" value="Udregn BMI">
    </label>
  <div class="calculation">
    <div>
      Dit BMI er: <span class="result"></span>
    </div>
    <div>
      Dette betyder at du er: <span class="health"></span>
    </div>
  </div>
</form>

Answer 1

Add the javascript after the closing tag of form

OR you can try this

window.onload = function() {
 var form = document.querySelector('form[name=bmi]');
 var onSubmit = function(event) {

event.preventDefault();
var healthMessage;

var result = form.querySelector('.result');
var health = form.querySelector('.health');

var weight = parseInt(form.querySelector('input[name=weight]').value, 10);
var height = parseInt(form.querySelector('input[name=height]').value, 10);

var bmi = (weight / (height / 100 * height / 100)).toFixed(1);

if (bmi < 18.5) {
  healthMessage = 'undervægtig';
} else if (bmi > 18.5 && bmi < 25) {
  healthMessage = 'normal vægtig';
} else if (bmi > 25) {
  healthMessage = 'overvægtig';
}
result.innerHTML = bmi;
health.innerHTML = healthMessage;
}
form.addEventListener('submit', onSubmit, false);
}