JSFiddle - React, Tailwind, and code Playground

HTML

<button id="groter">Groter</button>
<button id="kleiner">Kleiner</button>

<p>
    Een stuk tekst
</p>

JavaScript

function TextSize()
{
    var size = 16;
    var obj = {};
    obj.bigger = function()
    {
        size++;              
  		document.body.style.fontSize = size + 'px';
    }
    obj.smaller = function()
    {
        size--;        
        document.body.style.fontSize = size + 'px';
    }
    return obj;
}

var textSize = TextSize();
document.getElementById('groter').onclick = textSize.bigger;
document.getElementById('kleiner').onclick = textSize.smaller;