JavaScript:Set File Extension according to Files in folder -

i new in javascript doing image pagination,

function changedata(img)  {      image = document.getelementbyid('image');      var imagepath="images/"+img+".jpg";      image.src = imagepath; } 

here in changedata(img) passing image name , concatenate name image extension , set img src,

here 1 thing should keep in mind images copy , paste in images folder image name in sequence number 1,2,3,4.....etc

here did concatenation .jpg if 1 place images .png, .bmp, .gif create problem here how can .png, .bmp, .gif can 1 suggest me? please...

1.you pass extension variable function. 2.another way convert pictures .jpg. 3.(this worst way) since know format of each picture, can write this:

{      **var ext=".jpg";      if(img=="4"){ext=".png"}      else if ...**      image = document.getelementbyid('image');      var imagepath="images/"+img+**ext;**      image.src = imagepath;  }