How to checking size file in javascript

checkSize.html

<form method="post" enctype="multipart/form-data" onsubmit="return checkSize(2097152)">    
  <input type="file" id="upload">
  <input type="submit">
</form>
<script type="text/javascript">
function checkSize(max_img_size) {
    var input = document.getElementById("upload");
    if(input.files && input.files.length == 1) {           
        if (input.files[0].size > max_img_size) {
            alert("The file must be less than " + (max_img_size/1024/1024) + "MB");
            return false;
        }
    }
    return true;
}
</script>