this is link 1
this is link 2
this is link 3
this is link 4
this is link 5
this is link 6

To add this script to a file called boldLinks.js

  /* JavaScripts http://javascripts.com
  Created by: Calvin Leung | http://www.pphresh.com */
  function turnBackBold (speclink) {
  	 var anchors = document.links
  	 for (i=0; i<anchors.length; i++) {
      anchors[i].style.fontWeight = '300';
  	 }
  	 speclink.style.fontWeight = '900';
  }

Add this line of code to your <head> tag:

  <script type="text/javascript" src="boldLinks.js"></script>

Add this line of code to your <body> tag:

  <p>
  <a href="#" onclick="turnBackBold (this);">this is link 1</a><br>
  <a href="#" onclick="turnBackBold (this);">this is link 2</a><br>
  <a href="#" onclick="turnBackBold (this);">this is link 3</a><br>
  <a href="#" onclick="turnBackBold (this);">this is link 4</a><br>
  <a href="#" onclick="turnBackBold (this);">this is link 5</a><br>
  <a href="#" onclick="turnBackBold (this);">this is link 6</a></p>