Edit in JSFiddle

var swap = function (a, b) {
    var a, b;
    document.querySelector('.input').innerHTML = 'a = ' + a + '; b = ' + b;
    a = a + b;
    b = a - b;
    a = a - b;
    document.querySelector('.result').innerHTML = 'a = ' + a + '; b = ' + b;
}

swap(5, 10);

<h3>before swapping: <span class="input"></span></h3>
    
<h3>after swapping: <span class="result"></span></h3>
    
swapping algorithm:
<pre>
    a = a + b;
    b = a - b;
    a = a - b;
</pre>