Files: Making Your Program Remember

Every program in this book so far forgets everything the moment it ends. Chapter 16’s deck, chapter 14’s high scores, chapter 12’s reading list: all gone, every time.

A file fixes that. You already have the tool.

The same function, a real path

Chapter 5 read from the keyboard with fs.readFileSync(0, "utf-8"), where 0 meant “the keyboard”. Give it a filename instead and it reads a file:

/// <reference types="node" />
const fs = require("fs");

fs.writeFileSync("notes.txt", "first line\nsecond line\n");

const text = fs.readFileSync("notes.txt", "utf-8");
console.log(text);

first line
second line

The same /// <reference types="node" /> line from chapter 5, for the same reason. writeFileSync takes a path and some text and puts one in the other. readFileSync takes it back out, as one string with the newlines still in it, which is exactly what .split("\n") from chapter 5 is for.

Look in your folder and there’s a notes.txt sitting next to your .ts file. Open it in an editor. It’s an ordinary text file with nothing special about it, and that’s the point: your program’s memory is now something you can read, back up, and email to somebody.

When the file isn’t there

const text = fs.readFileSync("does-not-exist.txt", "utf-8");

Error: ENOENT: no such file or directory, open 'does-not-exist.txt'

Your program stops. Not undefined, not an empty string, a genuine crash with a stack trace.

That’s unusual for this book. Nearly every failure so far has been quiet: undefined from an array, NaN from Number(), a wrong answer with no complaint. This one is loud, and loud is easier.

ENOENT is short for “error: no entity”, and you’ll see it for the rest of your career. It nearly always means a typo in the path, or a file you expected somebody else to have created.

The fix is to look first:

if (fs.existsSync("notes.txt")) {
  const text = fs.readFileSync("notes.txt", "utf-8");
  console.log(text);
} else {
  console.log("No notes yet.");
}

A program that saves its own data needs exactly this on the very first run, when the file it’s about to write doesn’t exist yet.

Storing something other than text

A file holds text. Chapter 12’s objects are not text, so something has to translate.

type Task = { title: string; done: boolean };

const tasks: Task[] = [
  { title: "Write chapter", done: true },
  { title: "Verify it", done: false },
];

fs.writeFileSync("tasks.json", JSON.stringify(tasks, null, 2));
console.log(fs.readFileSync("tasks.json", "utf-8"));

[
  {
    "title": "Write chapter",
    "done": true
  },
  {
    "title": "Verify it",
    "done": false
  }
]

JSON.stringify turns a value into text. The null, 2 at the end is formatting, indent it by 2 spaces, and it’s worth the extra characters because the file stays readable by a human. Leave them off and you get one very long line, which is fine for a machine and miserable to debug.

JSON.parse goes the other way:

const loaded = JSON.parse(fs.readFileSync("tasks.json", "utf-8"));
console.log(loaded.length);
console.log(loaded[0].title);

2
Write chapter

Where TypeScript stops

Now the part of this chapter that matters most, and it’s uncomfortable.

Look again at what came back from JSON.parse:

const loaded = JSON.parse(fs.readFileSync("tasks.json", "utf-8"));
console.log(loaded.anything.at.all);

That compiles. No error, no warning, nothing.

JSON.parse returns any, chapter 3’s opt-out, because it genuinely cannot know what’s in a file it hasn’t read yet. And any means every guarantee you’ve built up since chapter 12 switches off at this line.

The obvious fix is to say what you expect:

const loaded: Task[] = JSON.parse(fs.readFileSync("tasks.json", "utf-8"));
console.log(loaded[0].titel);

error TS2551: Property 'titel' does not exist on type 'Task'.
Did you mean 'title'?

Checking is back. That looks like the problem solved, and it is worth doing, but be clear about what it actually did.

Paying chapter 11’s promise

Chapter 11 said console.log goes to stdout and console.error goes to stderr, and that the difference would matter here. It does, because both can be sent to files:

node report.js > out.txt 2> err.txt

out.txt:  real output
err.txt:  diagnostic

> redirects stdout, 2> redirects stderr, and they land in different places without your program knowing or caring. That’s why the two channels exist. A program can produce data on one and complaints on the other, and whoever runs it decides where each goes.

It also means a program can write a file without using fs at all, by printing and letting the shell catch it. Both are worth having.

type Task = { title: string; done: boolean };

printf 'add Buy milk\n' | node todo.js
printf 'list\n' | node todo.js
printf 'done 1\n' | node todo.js