How to read and write into file using JavaScript?

For completeness, the OP does not state he is looking to do this in a browser (if he is, as has been stated, it is generally not possible)

However javascript per se does allow this; it can be done with server side javascript.

See this documentation on the Javascript File class

Edit: That link was to the Sun docs that now have been moved by Oracle.

To keep up with the times here's the node.js documentation for the FileSystem class: http://nodejs.org/docs/latest/api/fs.html

Edit(2): You can read files client side now with HTML5: http://www.html5rocks.com/en/tutorials/file/dndfiles/

No. Browser-side javascript doesn't have permission to write to the client machine without a lot of security options having to be disabled

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.

If you are using JScript (Microsoft's Javascript) to do local scripting using WSH (NOT in a browser!) you can use Scripting.FileSystemObject to access the file system.

I think you can access that same object in IE if you turn a lot of security settings off, but that would be a very, very bad idea.

MSDN here

You'll have to turn to Flash, Java or Silverlight. In the case of Silverlight, you'll be looking at Isolated Storage. That will get you write to files in your own playground on the users disk. It won't let you write outside of your playground though.

Writing this answer for people who wants to get a file to download with specific content from javascript. I was struggling with the same thing.

const data = {name: 'Ronn', age: 27};              //sample json
const a = document.createElement('a');
const blob = new Blob([JSON.stringify(data)]);
a.href = URL.createObjectURL(blob);
a.download = 'sample-profile';                     //filename to download
a.click();

Check Blob documentation here - Blob MDN

You can't do this in any cross-browser way. IE does have methods to enable "trusted" applications to use ActiveX objects to read/write files, but that is it unfortunately.

If you are looking to save user information, you will most likely need to use cookies.

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!

You cannot do file i/o on the client side using javascript as that would be a security risk. You'd either have to get them to download and run an exe, or if the file is on your server, use AJAX and a server-side language such as PHP to do the i/o on serverside

There are two ways to read and write a file using JavaScript

1. Using JavaScript extensions

2. Using a web page and Active X objects