How do I use the File System Access API from Chrome using file:// url with no http server

File System API is not available currently in Chrome 85. For now you can launch the html file using a batch file which will locate and launch Chrome with the right appropriate command line options.

Name the batch file the same name as the html file, and place the following in the batch file:

@echo off

setlocal
set name=%~n0
set here=%~dp0

cd /d %here%
set indexFile=%here%%name%.html
if not exist "%indexFile%" set indexFile=%here%%name%.htm
if not exist "%indexFile%" Echo Could not locate "%name%.htm" or "%name%.html" & pause & goto :eof

get path to msedge.exe
set exe=
FOR /F "tokens=2* skip=2" %%a in ('reg query HKCR\MSEdgeHTM\DefaultIcon /ve') do set exe=%%b
cls
set exe=%exe:~0,-2%
if defined exe goto exeFound

rem get path to chrome.exe
set exe=
FOR /F "tokens=2* skip=2" %%a in ('reg query HKCR\ChromeHTML\DefaultIcon /ve') do set exe=%%b
cls
set exe=%exe:~0,-2%
if defined exe goto exeFound

start "" "%indexFile%"
goto :eof

:exeFound
start "" "%exe%" --enable-experimental-web-platform-features --disable-web-security --no-proxy-server --no-sandbox --allow-file-access-from-files --allow-file-access  --no-default-browser-check --no-first-run --allow-running-insecure-content --enable-local-file-accesses --disable-extensions --user-data-dir="%temp%\%name%" --app="file:///%indexFile%"

In the javascript you can make calls like this:

Determine if API is available

if (typeof showDirectoryPicker === 'undefined')

Access directory or file

const directoryHandle = await showDirectoryPicker()

const fileHandle = await directoryHandle.getFileHandle(fileName)

const str = await (await fileHandle.getFile()).text();

See showOpenFilePicker(), showSaveFilePicker() and showDirectoryPicker() at https://wicg.github.io/file-system-access/ for more.

Update:

Tested with Chrome 88, and the batch file is not needed at all anymore.

Update 2:

How to disable web security when launch Chrome or Edge against a file: based url, from c#:

private static void LaunchBrowser(string name, string indexFilePath)
{
    var exe = GetBrowserExePath();
    if (string.IsNullOrEmpty(exe))
    {
        Console.WriteLine("Could not locate browser");
        return;
    }

    var userDataDir = Path.Combine(Path.GetTempPath(), name);
    var indexFile = new Uri(indexFilePath).AbsoluteUri;

    var parameters =
        " --disable-web-security" +
        " --no-default-browser-check" +
        " --no-first-run" +
        " --disable-extensions" +
        " --user-data-dir=" + "\"" + userDataDir + "\"" +
        " --app=" + "\"" + indexFile + "\"" +
        "";
    Process.Start(exe, parameters);
}

private static string GetBrowserExePath()
{
    string exe = GetBrowserExePathWithKey("MdSEdgeHTM");
    if (string.IsNullOrEmpty(exe))
    {
        exe = GetBrowserExePathWithKey("ChromeHTML");
    }

    return exe;

    string GetBrowserExePathWithKey(string keyForChromeBaseBrowser)
    {
        var key = Registry.ClassesRoot.OpenSubKey(keyForChromeBaseBrowser);
        if (key != null)
        {
            key = key.OpenSubKey("DefaultIcon");
            if (key != null)
            {
                var v = key.GetValue(string.Empty) as string;
                var y = Regex.Match(v, @"(.*\.exe),");
                if (y.Success)
                {
                    var g = y.Groups;
                    if (g.Count > 1)
                    {
                        return g[1].Value;
                    }
                }
            }
        }

        return string.Empty;
    }
}