❓ ReferenceError: x is not defined

Common Causes

// ❌ Typo in variable name
const userName = 'Alice';
console.log(username);  // ReferenceError — capital N missing

// ❌ Used before declaration
console.log(myVar);  // ReferenceError
const myVar = 'hello';  // const/let are not hoisted like var

// ❌ Wrong scope
function greet() {
  const name = 'Alice';
}
console.log(name);  // ReferenceError — name is inside function scope

// ✅ var is hoisted (but still undefined, not an error)
console.log(myVar);  // undefined (not an error)
var myVar = 'hello';

Scope Issues

// Block scope with let/const
if (true) {
  let blockVar = 'inside';
}
console.log(blockVar);  // ❌ ReferenceError — outside block

// Function scope
function outer() {
  function inner() {
    const x = 10;
  }
  console.log(x);  // ❌ ReferenceError
}

// Closure — correct way to share
function outer() {
  const x = 10;
  function inner() {
    console.log(x);  // ✅ inner can access outer's variables
  }
  inner();
}

Module Import Issues

// ❌ Using global in module context
// In browser modules, there is no global jQuery unless imported
$('#btn').click(...);  // ReferenceError — $ is not defined

// ✅ Import it properly
import $ from 'jquery';

// ❌ Using a browser global in Node.js
console.log(window);  // ReferenceError in Node.js

// ❌ Circular dependency causing undefined
// A imports B, B imports A, one sees undefined