Importing values in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this ES module import?

Given the following module mathUtils.js exporting a constant, what will be logged by main.js?

Javascript

/* mathUtils.js */
export const pi = 3.14;

/* main.js */
import { pi } from './mathUtils.js';
console.log(pi);

A null

B undefined

C ReferenceError

D 3.14

💡 Hint

Remember that named exports can be imported with curly braces.

The constant pi is exported and imported correctly, so logging pi outputs 3.14.

❓ Predict Output

intermediate

What happens if you import a non-exported value?

Consider this module data.js and the import in app.js:

Javascript

/* data.js */
const secret = 'hidden';
export const visible = 'shown';

/* app.js */
import { secret } from './data.js';
console.log(secret);

A SyntaxError

B undefined

C TypeError

D hidden

💡 Hint

Only exported values can be imported.

Trying to import a non-exported value causes a syntax error because the export does not exist.

❓ Predict Output

advanced

What is the output when importing a default export?

Look at these two files:

Javascript

/* greet.js */
export default function greet() {
  return 'Hello!';
}

/* index.js */
import greet from './greet.js';
console.log(greet());

A undefined

B Hello!

C TypeError

D SyntaxError

💡 Hint

Default exports are imported without curly braces.

The default export is a function greet, which is imported and called correctly, printing 'Hello!'.

❓ Predict Output

advanced

What error occurs when mixing default and named imports incorrectly?

Given this module and import:

Javascript

/* utils.js */
export default function() { return 'default'; }
export const named = 'named';

/* app.js */
import { default as def, named } from './utils.js';
console.log(def(), named);

A ReferenceError

B TypeError

C default named

D SyntaxError

💡 Hint

You can rename default import using named import syntax.

The syntax imports the default export as 'def' and the named export 'named'. Calling def() returns 'default' and named is 'named'.

🧠 Conceptual

expert

Which option correctly imports all exports as an object?

You want to import everything from module.js as a single object named mod. Which import statement is correct?

A import * as mod from './module.js';

B import mod from './module.js';

C import { * as mod } from './module.js';

D import { mod } from './module.js';

💡 Hint

Use the star (*) syntax to import all exports as an object.

Option A correctly imports all exports as an object named mod. Option A imports the default export only. Option A and D are invalid syntax.

Practice

1. What is the correct way to import a named export calculate from a file math.js?

easy

A. import './math.js'.calculate;

B. import { calculate } from './math.js';

C. import * as calculate from './math.js';

D. import calculate from './math.js';

Solution

1. Step 1: Understand named imports syntax

   Named exports must be imported using curly braces around the exact exported name.

2. Step 2: Match syntax with example

   The syntax import { calculate } from './math.js'; correctly imports the named export calculate.

3. Final Answer:

   import { calculate } from './math.js'; -> Option B

4. Quick Check:

   Named import uses curly braces [OK]

Hint: Use curly braces for named imports, no braces for default [OK]

Common Mistakes:

• Using default import syntax for named exports
• Forgetting curly braces around named imports
• Trying to access named export as property after import

2. Which of the following is the correct syntax to import the default export from utils.js?

easy

A. import { default } from './utils.js';

B. import * as utils from './utils.js';

C. import utils from { './utils.js' };

D. import utils from './utils.js';

Solution

1. Step 1: Recall default import syntax

   Default exports are imported without curly braces using any name you choose.

2. Step 2: Identify correct syntax

   import utils from './utils.js'; correctly imports the default export.

3. Final Answer:

   import utils from './utils.js'; -> Option D

4. Quick Check:

   Default import no braces [OK]

Hint: Default imports have no braces, named imports do [OK]

Common Mistakes:

• Using curly braces for default import
• Incorrect braces placement
• Using * as for default import

3. Given export const name = 'Alice'; in person.js, what will be the output of this code?

import { name } from './person.js';
console.log(name);

medium

A. Alice

B. ReferenceError

C. undefined

D. null

Solution

1. Step 1: Understand named export and import

   The variable name is exported as a named export and imported correctly with curly braces.

2. Step 2: Check the logged value

   Since name is 'Alice', console.log(name); outputs 'Alice'.

3. Final Answer:

   Alice -> Option A

4. Quick Check:

   Named import logs exported value [OK]

Hint: Named import logs exported value directly [OK]

Common Mistakes:

• Forgetting curly braces causing ReferenceError
• Expecting default import without braces
• Confusing undefined with missing import

4. What is wrong with this import statement?

import { sum } from './math.js'
console.log(sum(2, 3));

Assuming sum is a default export in math.js.

medium

A. Missing semicolon after import

B. Wrong file path

C. Using curly braces for default export import

D. sum is not a function

Solution

1. Step 1: Identify export type

   The problem states sum is a default export, so it should be imported without curly braces.

2. Step 2: Check import syntax

   Using curly braces imports named exports, so import { sum } is incorrect for default export.

3. Final Answer:

   Using curly braces for default export import -> Option C

4. Quick Check:

   Default export no braces [OK]

Hint: Default export imports never use braces [OK]

Common Mistakes:

• Confusing default and named imports
• Ignoring export type in import
• Assuming semicolon is mandatory for correctness

5. You have two files:

// colors.js
export const red = '#FF0000';
export const blue = '#0000FF';
export default 'colors';

// app.js
import colors, { red, green } from './colors.js';
console.log(colors, red, green);

What will happen when running app.js?

hard

A. SyntaxError due to invalid import

B. Logs: colors #FF0000 undefined

C. Logs: undefined #FF0000 undefined

D. ReferenceError: green is not defined

Solution

1. Step 1: Analyze exports in colors.js

   There is a default export 'colors' and named exports 'red' and 'blue'. No 'green' export exists.

2. Step 2: Analyze import statement in app.js

   It imports default as colors and named red, but green does not exist, causing a SyntaxError during module loading.

3. Final Answer:

   SyntaxError due to invalid import -> Option A

4. Quick Check:

   Missing named export causes import error [OK]

Hint: Missing named exports cause import errors [OK]

Common Mistakes:

• Thinking missing named import becomes undefined
• Confusing default and named imports
• Expecting ReferenceError at runtime