Why "return false" doesn't work?

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Why in the code below return false doesn’t work at all?

<script>
  function handler() {
    alert( "..." );
    return false;
  }
</script>

<a href="/?originalUrl=https%3A%2F%2Ftr.javascript.info%2F%26quot%3Bhttp%3A%2F%2Fw3.org%26quot%3B%2520onclick%3D%26quot%3Bhandler()%26quot%3B%26gt%3Bthe%2520browser%2520will%2520go%2520to%2520w3.org%26lt%3B%2Fa%26gt%3B%253C%2Fcode">

The browser follows the URL on click, but we don’t want it.


How to fix?

When the browser reads the on* attribute like onclick, it creates the handler from its content.
For onclick="handler()" the function will be:

      
        
        
          function(event) {
  handler() // the content of onclick
}
        
      
      
      
Now we can see that the value returned by handler() is not used and does not affect the result.
The fix is simple:

      
        
        
          
            
          
          
            
          
        
        
          <script>
  function handler() {
    alert("...");
    return false;
  }
</script>

<a href="/?originalUrl=https%3A%2F%2Ftr.javascript.info%2F%26quot%3Bhttp%3A%2F%2Fw3.org%26quot%3B%2520onclick%3D%26quot%3Breturn%2520handler()%26quot%3B%26gt%3Bw3.org%26lt%3B%2Fa%26gt%3B%253C%2Fcode">

        

      

      
      
Also we can use event.preventDefault(), like this:


      
        
        
          
            
          
          
            
          
        
        
          <script>
  function handler(event) {
    alert("...");
    event.preventDefault();
  }
</script>

<a href="/?originalUrl=https%3A%2F%2Ftr.javascript.info%2F%26quot%3Bhttp%3A%2F%2Fw3.org%26quot%3B%2520onclick%3D%26quot%3Bhandler(event)%26quot%3B%26gt%3Bw3.org%26lt%3B%2Fa%26gt%3B%253C%2Fcode">