javascript中的拆分功能

Question

I want to change the extinsion of .doc to .txt.
alert(myvar) show empty alert.

enter code here
 <form>
 <input type="file" id="f1">
<button onclick="myFunction(f1.value)">Try it</button>
 </form>
  <script>
    function myFunction(a) {
      var mystr = a;
      var myarr = mystr.split(".doc");
      var ex= ".txt";
      var myvar = myarr + ex;
      alert(myvar);
       }
</script>

Answer 1

use str.replace function

mystr.replace(".doc", ".txt")