bun.sh/docs/guides/write-file/append.md

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Append content to a file

Bun implements the node:fs module, which includes the fs.appendFile and fs.appendFileSync functions for appending content to files.

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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");

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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!');
});

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To specify the encoding of the content:

import { appendFile } from "node:fs";

appendFile("message.txt", "data to append", "utf8", callback);

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To append the data synchronously, use fs.appendFileSync:

import { appendFileSync } from "node:fs";

appendFileSync("message.txt", "data to append", "utf8");

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See the Node.js documentation for more information.