Thomas Step

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5 April 2021

Reading and Writing JSON in JavaScript

by Thomas

File IO seems to be a topic that I normally end up Googling to find the right answer to. This last time I came across a convenient way to perform I told myself that I would instead contribute to this topic instead of only consume it. I recently needed to read and write JSON files and this is what I ended up coding and liking. Since it was one of the cleaner ways to handle this operation, I wanted to keep track of it. Here is an example script to write an object in Javascript as a JSON file.

const fs = require('fs');
const path = require('path');

const testObject = {
  hello: 'world',
  myArray: [
    'entry1',
    'entry2',
    'entry3',
  ],
  myNestedObject: {
    nestedHello: 'nestedWorld',
  },
};

const testJsonString = JSON.stringify(testObject, null, 2);

const filePath = path.join(process.cwd(), 'test.json');

fs.writeFile(filePath, testJsonString, (err) => {
  if (err) {
    console.error(err);
  } else {
    console.log('File written successfully');
  }
});

The JSON.stringify(testObject, null, 2) portion can be simplified to JSON.stringify(testObject) if you do not care about the output being pretty-printed. Here is an example script to read a JSON into an object.

const fs = require('fs');
const path = require('path');

const filePath = path.join(process.cwd(), 'test.json');

fs.readFile(filePath, 'utf8', (err, contents) => {
  if (err) {
    console.error(err);
    return;
  }

  try {
    const jsonString = JSON.parse(contents);
    console.log(jsonString);
  } catch (jsonError) {
    console.error('Error parsing JSON');
  }
});

The JSON is parsed in and printed in this script, and if you run the script that writes a file before the script that reads a file, then you will see an object with the same contents as the writing script printed in the reading script. Notice the 'utf8' parameter passed into the readFile call. That parameter is not necessary, but I wanted to include it here just to point out that files can be encoded differently. This is how we can tell Node what type of encoding to expect. A similar parameter can be passed into writeFile if desired.

Synchronous File IO

To write and read files synchronously in Node, we need to use writeFileSync and readFileSync. Instead of providing callbacks, the *Sync versions of these functions return only when they are done. There will be a hit on performance with this as file IO is not necessarily cheap. Here is the example script in a version that writes synchronously.

const fs = require('fs');
const path = require('path');

const testObject = {
  hello: 'world',
  myArray: [
    'entry1',
    'entry2',
    'entry3',
  ],
  myNestedObject: {
    nestedHello: 'nestedWorld',
  },
};

const testJsonString = JSON.stringify(testObject, null, 2);

const filePath = path.join(process.cwd(), 'test.json');

try {
  fs.writeFileSync(filePath, testJsonString);
} catch (err) {
  console.error(err);
}

Here is the example script in a version that reads synchronously.

const fs = require('fs');
const path = require('path');

const filePath = path.join(process.cwd(), 'test.json');

try {
  const contents = fs.readFileSync(filePath, 'utf8');
  const jsonString = JSON.parse(contents);
  console.log(jsonString);
} catch (err) {
  console.error(err);
}
Categories: dev | javascript