How to get the file size using Javascript in Chrome extensions?

How can I get the size of file written in a sandboxed sect开发者_JS百科ion of chrome using javascript ?

It is not amazingly easy.

The FileEntry and Entry interface has a method called getMetaData, but this doesn't include the file size. The only way that I could work out how to do it is as follows:

1. Get the file entry
2. Call file, to get a reference to the file, the file is of type Blob.
3. Read blob.size

It means potentially loading the file.