You can't. And thank God for that. Imagine how insecure the internet would've been if JS was able to access a client's file-system.
Of course, IE8 has the MS specific JScript superset (ActiveXObject), which does enable filesystem access:

var fileHandle,
    fs = new ActiveXObject("Scripting.FileSystemObject");
fileHandle = fs.OpenTextFile("C:\\path\\to\\file.tmp", 1, true);
console.log(fileHandle.ReadLine());

But this is non-standard, is - I think- no longer supported either, and doesn't work X-browser.
Here's the documentation. At the bottom there's a link to a more detailed overview of the properties and methods this object has to offer, as you can see, there's a lot to choose from

You can't. And thank God for that. Imagine how insecure the internet would've been if JS was able to access a client's file-system.
Of course, IE8 has the MS specific JScript superset (ActiveXObject), which does enable filesystem access:

var fileHandle,
    fs = new ActiveXObject("Scripting.FileSystemObject");
fileHandle = fs.OpenTextFile("C:\\path\\to\\file.tmp", 1, true);
fileHandle.Write('This is written to a file');
console.log(fileHandle.ReadLine());//will log what we've just written to the file

But this is non-standard, is - I think- no longer supported either, and doesn't work X-browser.
Here's the documentation. At the bottom there's a link to a more detailed overview of the properties and methods this object has to offer, as you can see, there's a lot to choose from