Question: Can anybody give some sample code to read and write a file using JavaScript?

Answer: 
here's the mozilla proposal

<http://www-archive.mozilla.org/js/js-file-object.html>

this is implemented with a compilation switch in spidermonkey, and also in adobe's extendscript. Additionally (I think) you get the File object in firefox extensions.

rhino has a (rather rudementary) readFile function
<https://developer.mozilla.org/en/Rhino_Shell>

for more complex file operations in rhino, you can use java.io.File methods.

you won't get any of this stuff in the browser though. For similar functionality in a browser you can use the SQL database functions from HTML5, clientside persistence, cookies, and flash storage objects.
From a ReactJS test, the following code successfully writes a file:

```
import writeJsonFile from 'write-json-file';

const ans = 42;
writeJsonFile('answer.txt', ans);

const json = {"answer": ans};
writeJsonFile('answer_json.txt', json);

```

The file is written to the directory containing the tests, so writing to an actual JSON file '\*.json' creates a loop!