Module scope in Javascript - Mini Project: Build & Apply

Understanding Module Scope in JavaScript

📖 Scenario: You are building a small JavaScript module to manage a user's profile data. You want to keep some data private inside the module and expose only what is necessary.

🎯 Goal: Create a JavaScript module that stores a user's name privately and exposes a function to get the user's name.

📋 What You'll Learn

Create a module using an immediately invoked function expression (IIFE).

Inside the module, create a private variable called userName with the value 'Alice'.

Add a function called getUserName inside the module that returns the value of userName.

Expose only the getUserName function to the outside of the module.

Print the result of calling getUserName() to the console.

💡 Why This Matters

🌍 Real World

Modules help keep code organized and protect data from being changed accidentally by other parts of a program.

💼 Career

Understanding module scope is important for writing clean, maintainable JavaScript code in real projects and working with libraries.

1

Create the module and private variable

Create a module using an immediately invoked function expression (IIFE). Inside it, create a private variable called userName and set it to 'Alice'.

Javascript

(function() {
  // Your code here
})();

Hint

Use (function() { ... })(); to create a module. Declare const userName = 'Alice' inside it.

2

Add a function to get the user name

Inside the module, add a function called getUserName that returns the value of userName.

Javascript

(function() {
  const userName = 'Alice';
  // Your code here
})();

Hint

Define function getUserName() { return userName; } inside the module.

3

Expose the getUserName function

Modify the module to expose only the getUserName function to the outside by returning an object with getUserName as a property.

Javascript

(function() {
  const userName = 'Alice';
  function getUserName() {
    return userName;
  }
  // Your code here
})();

Hint

Return an object with getUserName inside the IIFE and assign it to userModule.

4

Print the user name using the module

Use console.log to print the result of calling userModule.getUserName().

Javascript

const userModule = (function() {
  const userName = 'Alice';
  function getUserName() {
    return userName;
  }
  return { getUserName };
})();

// Your code here

Hint

Call console.log(userModule.getUserName()) to show the user name.

Practice

1. What does module scope mean in JavaScript?

easy

A. All variables are global and accessible everywhere

B. Variables and functions are private to the file unless exported

C. Functions can only be used inside loops

D. Variables must be declared with var only

Solution

1. Step 1: Understand module scope concept

   Module scope means code inside a file is private unless explicitly shared.

2. Step 2: Compare options with this concept

   Only Variables and functions are private to the file unless exported correctly states variables/functions are private unless exported.

3. Final Answer:

   Variables and functions are private to the file unless exported -> Option B

4. Quick Check:

   Module scope = private unless exported [OK]

Hint: Remember: module code is private unless exported [OK]

Common Mistakes:

• Thinking all variables are global
• Confusing module scope with block scope
• Believing functions must be inside loops

2. Which of the following is the correct way to export a function named greet from a module?

easy

A. export function greet() {}

B. function export greet() {}

C. export greet() {}

D. export = greet()

Solution

1. Step 1: Recall correct export syntax

   To export a function, use export function functionName() {}.

2. Step 2: Check each option

   export function greet() {} matches the correct syntax; others are invalid or incorrect.

3. Final Answer:

   export function greet() {} -> Option A

4. Quick Check:

   Export function syntax = export function name() [OK]

Hint: Use 'export function' before function name [OK]

Common Mistakes:

• Placing export after function keyword
• Using incorrect export assignment syntax
• Missing parentheses in function declaration

3. Given this module code:

const secret = 42;
export const visible = 'hello';

console.log(typeof secret);
console.log(typeof visible);

What is the output when this module runs?

medium

A. "number" and "string"

B. "undefined" and "string"

C. "number" and "undefined"

D. ReferenceError for secret

Solution

1. Step 1: Understand variable scope and export

   Both secret and visible exist inside the module; secret is private, visible is exported.

2. Step 2: Check typeof outputs inside the same module

   Inside the module, both variables exist, so typeof secret is "number" and typeof visible is "string".

3. Final Answer:

   "number" and "string" -> Option A

4. Quick Check:

   typeof secret = number, typeof visible = string [OK]

Hint: Inside module, all declared variables exist [OK]

Common Mistakes:

• Thinking private variables are undefined inside module
• Confusing export with variable existence
• Expecting ReferenceError inside same file

4. What is wrong with this module code?

const count = 10;
export count = 20;

medium

A. Cannot export a variable twice

B. count is not declared before export

C. Syntax error: export cannot be used with assignment

D. No error, code works fine

Solution

1. Step 1: Analyze export syntax

   Export keyword cannot be used with assignment like export count = 20;. It must be export const count = 20; or export existing variables.

2. Step 2: Identify error type

   This causes a syntax error because export expects declaration or named export, not assignment.

3. Final Answer:

   Syntax error: export cannot be used with assignment -> Option C

4. Quick Check:

   Export syntax must declare or export existing variables [OK]

Hint: Export must declare or export, not assign [OK]

Common Mistakes:

• Trying to export by assignment
• Missing const/let/var with export
• Assuming export can change variable value

5. You have two modules:

// file1.js
const secret = 'hidden';
export const data = 123;

// file2.js
import { data } from './file1.js';
console.log(secret);
console.log(data);

What will happen when file2.js runs?

hard

A. Logs undefined and 123

B. Logs 'hidden' and 123

C. SyntaxError due to import

D. ReferenceError: secret is not defined

Solution

1. Step 1: Understand module scope and imports

   The variable secret is private to file1.js and not exported, so it cannot be accessed in file2.js.

2. Step 2: Predict runtime behavior

   Trying to access secret in file2.js causes a ReferenceError. The imported data logs correctly.

3. Final Answer:

   ReferenceError: secret is not defined -> Option D

4. Quick Check:

   Private variables cause ReferenceError outside module [OK]

Hint: Only exported variables are accessible in other modules [OK]

Common Mistakes:

• Assuming private variables are imported automatically
• Confusing import syntax errors with runtime errors
• Expecting undefined instead of ReferenceError