How to print out line in Javascript -

i want test return value of function getfileextension(input.files[0].name) (i have comment there point @ line) question how print out value in javascript? thanks!

<script>     function getfileextension(filename) {         var ext=filename.split('.').pop();         return ext     } </script>  <input type='file' onchange="readurl(this);">   <script> function readurl(input) {     if (input.files && input.files[0]) {         if (getfileextension(input.files[0].name)=="png") { // line our problem              var reader = new filereader();             reader.onload = function (e) {                 document.getelementbyid("pdf").innerhtml="<img id='blah' src=" +                                        e.target.result + " alt='your image' width='450'>"             }             reader.readasdataurl(input.files[0]);         }     } } </script> 

you can print out values in javascript writing

console.log(value); 

most browsers have console can see pressing f12. values end there.

if using internet explorer remember have developer tools open (f12) or error.