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Bun implements the node:fs module, which includes the fs.appendFile and fs.appendFileSync functions for appending content to files.
You can use fs.appendFile to asynchronously append data to a file, creating the file if it does not yet exist. The content can be a string or a Buffer. 
import {appendFile} from 'node:fs/promises';

await appendFile('message.txt', 'data to append');
To use the non-Promise API: 
import {appendFile} from 'node:fs';

appendFile('message.txt', 'data to append', err => {
	if (err) throw err;
	console.log('The "data to append" was appended to file!');
});
To specify the encoding of the content: 
import {appendFile} from 'node:fs';

appendFile('message.txt', 'data to append', 'utf8', callback);
To append the data synchronously, use fs.appendFileSync: 
import {appendFileSync} from 'node:fs';

appendFileSync('message.txt', 'data to append', 'utf8');
See the Node.js documentation for more information.