Module scope in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is module scope in JavaScript?

Module scope means variables and functions declared inside a module are only accessible within that module, not outside it.

beginner

How do you create a module in JavaScript?

You create a module by saving code in a separate file and using import and export statements to share parts of it.

beginner

What happens if you declare a variable inside a module without exporting it?

That variable stays private to the module and cannot be accessed from other modules.

beginner

Why is module scope useful?

It helps keep code organized and avoids name conflicts by hiding variables and functions inside modules.

beginner

How do you export a function from a module?

Use <code>export function myFunc() {}</code> or <code>export { myFunc }</code> to make it available outside the module.

What does module scope restrict?

A Access to variables and functions outside the module

B Access to global variables

C Access to browser APIs

D Access to CSS styles

Module scope restricts access to variables and functions declared inside the module from outside it.

Which keyword is used to share code from a module?

A import

B export

C var

D let

The export keyword shares code from a module so other modules can import it.

If a variable is not exported, can other modules use it?

A Only if declared with var

B Yes, always

C No, it stays private

D Only if imported

Variables not exported remain private to their module and cannot be accessed from outside.

How do you import a function named foo from a module?

A var foo = require('./module.js');

B export foo from './module.js';

C let foo = import('./module.js');

D import foo from './module.js';

Use import foo from './module.js'; to import the default exported function foo.

What is a benefit of module scope?

A It hides variables to avoid conflicts

B It slows down code

C It disables functions

D It makes all variables global

Module scope hides variables and functions inside modules to avoid name conflicts and keep code organized.

Explain what module scope is and why it is important in JavaScript.

Think about how modules keep code separate and private.

Describe how you share and use code between JavaScript modules.

Focus on how to make code available outside a module.

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