A Promise Result

Part of the Node.js Basics section of Coddy's Node.js journey. Lesson 19 of 33.

A promise represents a result that will arrive later. Unlike the callback version from node:fs, readFile from node:fs/promises takes no callback and returns a promise. Its .then method accepts a function to call with the result when the read succeeds.

import { readFile } from 'node:fs/promises';
readFile('name.txt', 'utf8').then((name) => {
    console.log('Hello, ' + name);
});

The name parameter holds the file text, not a promise. Work that depends on the text belongs here. For now the file is provided and the read succeeds. The following step will handle a read that fails.

Challenge

Easy

Run practice.js first: it prints Note: [object Promise]. The program is combining text with the promise instead of its eventual result.

Use a .then callback to print the completed read as Note: followed by the file text. No argument reads note.txt, giving Note: Buy milk. Try other.txt: expect Note: Call Ada.

Try it yourself

import { readFile } from 'node:fs/promises';
const file = process.argv[2] ?? 'note.txt';
console.log('Note: ' + readFile(file, 'utf8'));

All lessons in Node.js Basics

1Running JavaScript on Node

Hello, Node Reading the Command Command-Line Arguments Commands

Errors and Exit Codes

Environment Variables

4Asynchronous Node

Using a Callback Result CallbacksA Promise ResultWhen a Promise Fails Promises Waiting for a Value Async and Await Two Independent Reads Promise.all The Event Loop Timers

2Modules

5Events, Input and Packages

Listen Before Emitting Event Emitters Reading Input

package.json

3Files

JSON Text and Values Reading a File Writing JSON Text Writing a File Removing a Note

Handling Errors

6Project: Search and Tags