How to read and write a JSON file in Node.js

Short answer

Read the file as a string and parse it with JSON.parse():

import { readFile, writeFile } from 'node:fs/promises';
 
await writeFile('settings.json', '{"theme":"dark","fontSize":14}');
 
const settings = JSON.parse(await readFile('settings.json', 'utf8'));
console.log(settings.theme, settings.fontSize);

Output:

dark 14

Writing

JSON.stringify() with an indent of 2 makes the file readable:

import { readFile, writeFile } from 'node:fs/promises';
 
const settings = { theme: 'light', languages: ['en', 'de'] };
await writeFile('settings.json', JSON.stringify(settings, null, 2));
 
console.log(await readFile('settings.json', 'utf8'));

Output:

{
  "theme": "light",
  "languages": [
    "en",
    "de"
  ]
}

Update a value

Read, change and write back:

import { readFile, writeFile } from 'node:fs/promises';
 
await writeFile('counter.json', '{"visits":41}');
 
const data = JSON.parse(await readFile('counter.json', 'utf8'));
data.visits++;
await writeFile('counter.json', JSON.stringify(data));
 
console.log(await readFile('counter.json', 'utf8'));

Output:

{"visits":42}

Common mistake

A damaged file makes JSON.parse() throw. Catch the error and use default values:

import { readFile, writeFile } from 'node:fs/promises';
 
await writeFile('broken.json', '{"theme": "dark",}');
 
let settings;
try {
  settings = JSON.parse(await readFile('broken.json', 'utf8'));
} catch (error) {
  console.log('Broken file:', error.name);
  settings = { theme: 'light' };
}
console.log(settings.theme);

Output:

Broken file: SyntaxError
light

Thanks for reading!

Short answer

Writing

Update a value

Common mistake

Functions used in this recipe

Thanks for reading!