How to fix ReferenceError: variable is not defined in Node.js

// Incorrect - variable not declared
console.log(userName); // ReferenceError: userName is not defined

// Correct - declare before use
const userName = "John";
console.log(userName); // Works: "John"

const userName = "Jane";

// Incorrect - wrong capitalization
console.log(username); // ReferenceError: username is not defined

// Correct - exact match
console.log(userName); // Works: "Jane"

function setupUser() {
  const userId = 123;
  console.log(userId); // Works: 123
}

setupUser();
console.log(userId); // ReferenceError: userId is not defined

// Solution: declare variable in outer scope
let userId;
function setupUser() {
  userId = 123;
}

setupUser();
console.log(userId); // Works: 123

// Incorrect - accessing before declaration
console.log(status); // ReferenceError: Cannot access 'status' before initialization
const status = "active";

// Correct - declare before use
const status = "active";
console.log(status); // Works: "active"

// file: utils.js
export const API_URL = "https://api.example.com";

// file: app.js (incorrect)
console.log(API_URL); // ReferenceError: API_URL is not defined

// file: app.js (correct)
import { API_URL } from "./utils.js";
console.log(API_URL); // Works

const { API_URL } = require("./utils.js");

npm install --save-dev eslint
npx eslint --init

{
  "rules": {
    "no-undef": "error"
  }
}

npx eslint .