Reading and Writing Files

Node can read configuration, logs and data files. The promises API keeps asynchronous file work readable.

Read

Use readFile with the utf8 encoding to get a string rather than a buffer.

import { readFile } from "node:fs/promises";

try {
  const text = await readFile("notes.txt", "utf8");
  console.log(text);
} catch (error) {
  console.error("Could not read notes.txt:", error.message);
}

Write and Append

writeFile replaces the file. appendFile adds to the end, which is useful for logs.

import { writeFile, appendFile } from "node:fs/promises";

await writeFile("output.txt", "First line\n");
await appendFile("output.txt", "Second line\n");

JSON

Read the file, parse it, and handle a parse error rather than crashing.

import { readFile, writeFile } from "node:fs/promises";

async function loadConfig() {
  try {
    const text = await readFile("config.json", "utf8");
    return JSON.parse(text);
  } catch (error) {
    if (error.code === "ENOENT") return {};
    throw error;
  }
}

const config = await loadConfig();
await writeFile("config.json", JSON.stringify(config, null, 2));

Practice

1. Read a text file and print the number of lines.
2. Append a timestamp to a log file.
3. Load JSON settings and provide defaults when the file is missing.

Hands-on exercise

Write a script that reads notes.txt and prints how many lines it contains.
If the file is missing, print 'No notes file found.'

Recap

The promises API makes file work readable. Handle missing files deliberately and never trust user paths.