Getting Started with JSONL

Method 1: Standard Library (Recommended)

import json

# Read line by line - memory efficient!
with open('users.jsonl', 'r') as file:
    for line in file:
        user = json.loads(line)
        print(f"Name: {user['name']}, Role: {user['role']}")

# Output:
# Name: Alice Johnson, Role: developer
# Name: Bob Smith, Role: designer
# Name: Charlie Brown, Role: manager

Method 2: Load All Data

import json

# Read all at once (use only for small files)
users = []
with open('users.jsonl', 'r') as file:
    for line in file:
        users.append(json.loads(line))

print(f"Total users: {len(users)}")  # Total users: 3

Using readline module

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

async function readJSONL(filename) {
    const fileStream = fs.createReadStream(filename);
    const rl = readline.createInterface({
        input: fileStream,
        crlfDelay: Infinity
    });

    for await (const line of rl) {
        const user = JSON.parse(line);
        console.log(`Name: ${user.name}, Role: ${user.role}`);
    }
}

readJSONL('users.jsonl');

Using bufio.Scanner

package main

import (
    "bufio"
    "encoding/json"
    "fmt"
    "os"
)

type User struct {
    ID    int    `json:"id"`
    Name  string `json:"name"`
    Email string `json:"email"`
    Role  string `json:"role"`
}

func main() {
    file, _ := os.Open("users.jsonl")
    defer file.Close()

    scanner := bufio.NewScanner(file)
    for scanner.Scan() {
        var user User
        json.Unmarshal(scanner.Bytes(), &user)
        fmt.Printf("Name: %s, Role: %s\n", user.Name, user.Role)
    }
}

Kept short to show the shape. Two things to add before this reads real data: bufio.Scanner refuses any line longer than 64KB and then stops quietly, so call scanner.Buffer to raise the limit and check scanner.Err() after the loop. The examples page shows the hardened version.

import json

users = [
    {"id": 1, "name": "Alice Johnson", "role": "developer"},
    {"id": 2, "name": "Bob Smith", "role": "designer"},
    {"id": 3, "name": "Charlie Brown", "role": "manager"}
]

# Write to JSONL file
with open('output.jsonl', 'w') as file:
    for user in users:
        # Write each object as JSON on its own line
        file.write(json.dumps(user) + '\n')

print("File written successfully!")

const fs = require('fs');

const users = [
    {id: 1, name: "Alice Johnson", role: "developer"},
    {id: 2, name: "Bob Smith", role: "designer"},
    {id: 3, name: "Charlie Brown", role: "manager"}
];

// Create write stream
const writeStream = fs.createWriteStream('output.jsonl');

// Write each object
users.forEach(user => {
    writeStream.write(JSON.stringify(user) + '\n');
});

writeStream.end();
console.log('File written successfully!');