Javascript: click event handler not working

Specify font size in pixels inside string instead of using number 20. Also move the script tag to bottom of body tag.

Please find the working snippit below:

    function check()
    {
        var para=document.getElementsByTagName("p");
        para[0].style.fontSize= '20px' ;
    }

    <p>This a paragraph</p>
    <p>This a paragraph</p>
    <p>This a paragraph</p>
    <p>This a paragraph</p>
    <button onclick="check()">Check</button>

For each Tag P must use like this...

js

function check() {
  var s=document.getElementsByTagName('p');
  for(i=0;i<s.length;i++)
  {
    s[i].setAttribute("style","font-size:20px");
  }
}

View

<p>This a paragraph</p>
<p>This a paragraph</p>
<p>This a paragraph</p>
<p>This a paragraph</p>
<button onClick="check()">Check</button>

you have to define the size in string with px,em,% behind and wrap it with quote

para[0].style.fontSize = 20;

to

para[0].style.fontSize = "20px";