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Preventing Default Behaviour

You can stop the default action of an HTML element by using event.preventDefault(). The default action can also be stopped by executing 'return false;' on the inline event attribute.

In this example, clicking the link or the checkbox won't change or trigger anything. Only small letters can be entered in the textbox.
RESETRUNFULL
<!DOCTYPE html><html><head>
   <script>
      function clk(e){e.preventDefault();}
          function prs(e){
         var charCode = e.charCode;
         if (charCode < 97 || charCode > 122) {
            e.preventDefault();
            alert(
                "Please use lowercase letters only."
                + "\n" + "charCode: " + charCode + "\n"
            );
         }
      }
   </script></head><body>
   <a href="http://google.com" onclick="return false;">
         Google</a>
   <input type='checkbox' onclick="clk(event)"/>
   <input type='text' onkeypress="prs(event)"/></body></html>

If you define the event handler programmatically, such as by using .addEventListener(), you cannot prevent the default behavior by running 'return false;' in the handler function, as you do inside an inline event attribute. In such cases, event.preventDefault()remains the sole method to stop the default behavior.