how to get txt file size uisng js

Question

i want to ask how can i detect the file size before i upload to server. i have here an example i found it on the net. but the problem it detects only the size of an image file what i need is to validate any files specially txt files. If you have any idea pls help.. thanks function showSize(f) { if ((new Image()).fileSize) { var img = new Image(); img.onload = imgOnload; img.onerror = imgOnerror; img.src = f.elements['myFile'].value; } } function imgOnload() { if (this.fileSize) { var size = this.fileSize; if (size > 1024) { size = Math.round((size / 1024) * 100) / 100; } document.write( 'The file is ' + size + (this.fileSize > 1024 ? ' kilobytes' : ' bytes') + ' in size.' ); } } function imgOnerror() { alert('The file is not an image or did not load.'); }