var Emaillist = new File("emaillist.dat") returns error

[Matt_17]

I am writing a web site and I am using javascript to read/write to a file. There are multiple sites that say to do this:

var file = new File(txtFile);

however, it doesn't work. I fired up chrome's developer tools and went to the javascript console, and it gave me this:
Uncaught TypeError: Failed to construct 'File': The 1st argument provided is either null, or an invalid Array object

please help quick, my deadline is june 15

File is readonly. Take a look at this tutorial: http://www.html5rocks.com/en/tutorials/file/dndfiles/