How to overwrite a file in Chrome App?

Question

I followed this example:

chrome.fileSystem.chooseEntry({type:'openDirectory'}, function(entry) {
    chrome.fileSystem.getWritableEntry(entry, function(entry) {
        entry.getFile('file1.txt', {create:true}, function(entry) {
            entry.createWriter(function(writer) {
                writer.write(new Blob(['Lorem'], {type: 'text/plain'}));
            });
        });
        entry.getFile('file2.txt', {create:true}, function(entry) {
            entry.createWriter(function(writer) {
                writer.write(new Blob(['Ipsum'], {type: 'text/plain'}));
            });
        });
    });
});

to overwrite some existing file file1.txt and file2.txt.

But I found a problem: if the files are not empty, their content won't be fully overwritten, only the beginning part will be overwritten.

Do I need to remove the files first? Or do I miss something?

Answer 1

It looks like write only overwrites the contents of the file at the specified position, so you are correct that if you want to completely replace the text of the file, you'd need to either remove the files first or truncate them.

This code worked for me, by truncating the file to the position of the writer after the write is finished.

chrome.fileSystem.chooseEntry({type:'openDirectory'}, function(entry) {
    chrome.fileSystem.getWritableEntry(entry, function(entry) {
        entry.getFile('file1.txt', {create:true}, function(entry) {
            entry.createWriter(function(writer) {
                writer.onwriteend = function(e) {
                    e.currentTarget.truncate(e.currentTarget.position);
                };
                writer.write(new Blob(['Lorem'], {type: 'text/plain'}));
            });
        });
        entry.getFile('file2.txt', {create:true}, function(entry) {
            entry.createWriter(function(writer) {
                writer.onwriteend = function(e) {
                    e.currentTarget.truncate(e.currentTarget.position);
                };
                writer.write(new Blob(['Ipsum'], {type: 'text/plain'}));
            });
        });
    });
});