The File Writer API is defunct and never saw significant browser support.
You cannot write files from browser-based JavaScript. What you do instead is provide the user with a link that they can download, like this:
var filename = "readme.txt";
var text = "Text of the file goes here.";
var blob = new Blob([text], {type:'text/plain'});
var link = document.createElement("a");
link.download = filename;
link.innerHTML = "Download File";
link.href = window.URL.createObjectURL(blob);
document.body.appendChild(link);
That works on browsers that support the File API (which modern ones do, but not IE9 or earlier).
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