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Below is the sample HTML and JS that I created for the problem that I am having. I have a form that has three inputs. The value from the first two inputs will dictate the value of the third. I also want the values of all three inputs to display in <p> tags outside of the form. I have been able to update the first two <p> tags outside the form correctly, and I have been able to update the third input value correctly. However, I cannot get the third <p> tag to update when the third input value has dynamically changed.

Here is a Codepen for the problem.

HTML

<form>
 <input id="one" type="number" class="sumVari" name="a"> 
 <input id="two" type="number" class="sumVari" name="b">
 <input id="three" type="number" name="sum" placeholder="sum of a & b">
</form>
<div id="displayValues">
 <p>The value of a is: <span id="a"></span> </p>
 <p>The value of a is: <span id="b"></span></p>
 <p>The value of the sum is: <span id="sum"></span></p>
</div>

JS

let one = $('#one');
let two = $('#two');
let three = $('#three');

$('form').on('change', 'input', function() 
 let target = $(this);
 let attrName = target.attr('name');

 $('#'+attrName).text(target.val());
);

function sum(a,b) 
 return a+b;


$('.sumVari').each(function()
 $(this).change(function()
 if(one.val() != '' && two.val() != '') 
 three.val(sum(one.val(), two.val()));
 
 );
);

1. Call the change event manually when putting value on the sum

let one = $('#one');
let two = $('#two');
let three = $('#three');

$('form').on('change', 'input', function() 
 let target = $(this);
 let attrName = target.attr('name');

 $('#' + attrName).text(target.val());
);

function sum(a, b) 
 return a + b;


$('.sumVari').each(function() 
 $(this).change(function() 
 if (one.val() != '' && two.val() != '') 
 three.val(sum(one.val(), two.val())).change();//call the change event manually here on the sum input so that the change function will run on the sum input
 
 );
);

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<form>
 <input id="one" type="number" class="sumVari" name="a">
 <input id="two" type="number" class="sumVari" name="b">
 <input id="three" type="number" name="sum" placeholder="sum of a & b">
</form>
<div id="displayValues">
 <p>The value of a is: <span id="a"></span> </p>
 <p>The value of a is: <span id="b"></span></p>
 <p>The value of the sum is: <span id="sum"></span></p>
</div>