javascript中的拆分功能

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>

use str.replace function

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

function changeExt(s){
    if(s.substring(s.length - 3,s.length) == "doc"){
        var s = s.substring(0,s.length - 3);
        s += "txt";
        return s;
    }
    else{
        return 'bad ext';
    }

}

you have array whwn used split function . so

<script>
    function myFunction(a) {
      var mystr = a;
      var myarr = mystr.split(".doc");
      var ex= ".txt";
      var myvar = myarr[0] + ex;
      alert(myvar);
       }
</script>

 <script>
    function myFunction(a) {
      var mystr = a;
      var myarr = mystr.split(".doc")[0];
      var ex= ".txt";
      var myvar = myarr + ex;
      alert(myvar);
       }
</script>

You can do the following

 <input type="file" id="f1">
<input type="button" value="test" onclick="test(f1.value)">


function test(a){
    var file = a;
file = file.split(".");
file = file[0]+".html"; //any extension you want
 alert(file);   
}

please try the given below code.also follow the FIDDLE DEMO

In html:
<input type="file" id="f1">
<input type="button" value="myfun" onclick="myFunction(f1.value)">

In js:

window.myFunction = function(a) {
 var file = a;
 file = file.split(".");
 file = file[0]+".txt"; 
 alert(file);   

}

please try the given below code.also follow the FIDDLE DEMO

In html:
<input type="file" id="f1">
<input type="button" value="myfun" onclick="myFunction(f1.value)">

In js:

window.myFunction = function(fileName) {
    var newFileName = fileName.substr(0, fileName.lastIndexOf("."));
    newFileName = newFileName+".txt"; 
     alert(newFileName);   
}

why you choose spilit function when jquery provide us replace string and its variable too. here is example

  var str = "Prabhash Rawat";
  var res = str.replace("Rawat", "Huda");

after run this example the output of this script will be

Prabhash Huda