Default exports in Javascript - Mini Project: Build & Apply

Default exports

📖 Scenario: You are building a small JavaScript module system where one module exports a default function, and another module imports and uses it.

🎯 Goal: Create a module that exports a default function, then import and use that function in another file.

📋 What You'll Learn

Create a JavaScript file named greet.js that exports a default function called greet.

The greet function should return the string 'Hello, world!'.

Create another JavaScript file named app.js that imports the default export from greet.js.

Call the imported function and print its return value using console.log.

💡 Why This Matters

🌍 Real World

Default exports are used to share one main value or function from a JavaScript module, making it easy to import and use in other parts of an application.

💼 Career

Understanding default exports is essential for working with modern JavaScript frameworks and libraries, as well as organizing code in professional projects.

1

Create the default export function

Create a file named greet.js and write a default export function called greet that returns the string 'Hello, world!'.

Javascript

// Your code here

Hint

Use export default function greet() { return 'Hello, world!'; } to export the function.

2

Import the default export

In a new file named app.js, import the default export from greet.js using the name greet.

Javascript

// Your code here

Hint

Use import greet from './greet.js'; to import the default export.

3

Call the imported function

In app.js, call the imported greet function and store the result in a variable named message.

Javascript

import greet from './greet.js';

// Your code here

Hint

Call the function with greet() and assign it to message.

4

Print the message

In app.js, print the message variable using console.log.

Javascript

import greet from './greet.js';

const message = greet();

// Your code here

Hint

Use console.log(message); to print the message.

Practice

1. What is a default export in JavaScript modules?

easy

A. A single main value or function exported from a file without curly braces when importing.

B. Multiple named values exported from a file using curly braces when importing.

C. A way to export variables only, not functions.

D. A special export that requires double curly braces when importing.

Solution

1. Step 1: Understand export types

   JavaScript modules can export multiple named items or one default item.

2. Step 2: Identify default export usage

   Default export is a single main export imported without curly braces.

3. Final Answer:

   A single main value or function exported from a file without curly braces when importing. -> Option A

4. Quick Check:

   Default export = single main export without braces [OK]

Hint: Default exports import without braces, named exports need braces [OK]

Common Mistakes:

• Confusing default export with named exports
• Using curly braces when importing default export
• Thinking default export can export multiple items

2. Which of the following is the correct syntax to export a default function named greet?

easy

A. default export function greet() {}

B. export function default greet() {}

C. export default function greet() {}

D. export greet default function() {}

Solution

1. Step 1: Recall default export syntax

   The correct syntax is to write export default followed by the function declaration.

2. Step 2: Check each option

   Only export default function greet() {} matches the correct syntax: export default function greet() {}.

3. Final Answer:

   export default function greet() {} -> Option C

4. Quick Check:

   export default + function name = correct syntax [OK]

Hint: Remember: 'export default' comes before function name [OK]

Common Mistakes:

• Placing 'default' after function name
• Using 'default export' instead of 'export default'
• Mixing order of keywords

3. Given the file math.js with content:

export default function add(a, b) { return a + b; }

What will be the output of this code?

import add from './math.js';
console.log(add(3, 4));

medium

A. undefined

B. 7

C. TypeError

D. NaN

Solution

1. Step 1: Understand import of default export

   The function add is exported as default and imported without braces correctly.

2. Step 2: Evaluate function call

   Calling add(3, 4) returns 3 + 4 = 7, which is logged.

3. Final Answer:

   7 -> Option B

4. Quick Check:

   Default import calls function correctly = 7 [OK]

Hint: Default export imports as named variable, call normally [OK]

Common Mistakes:

• Using curly braces when importing default export
• Expecting undefined because of wrong import
• Confusing default export with named export

4. Identify the error in this code snippet:

import { greet } from './hello.js';
export default function greet() { return 'Hi'; }

medium

A. Cannot import default export using curly braces.

B. Function cannot be named greet when exported default.

C. Missing semicolon after export statement.

D. Default export must be a variable, not a function.

Solution

1. Step 1: Check import syntax

   Importing default export must be done without curly braces, but here curly braces are used.

2. Step 2: Confirm export syntax

   Exporting default function named greet is valid, so no error there.

3. Final Answer:

   Cannot import default export using curly braces. -> Option A

4. Quick Check:

   Default import no braces, braces cause error [OK]

Hint: Default exports import without braces; braces mean named import [OK]

Common Mistakes:

• Using curly braces to import default export
• Thinking default export must be unnamed
• Assuming semicolon is mandatory for export

5. You have two files:

// file1.js
const value = 42;
export default value;

// file2.js
import val from './file1.js';
console.log(val);

How can you change file1.js to export both value as default and a named constant name with value 'Answer'?

hard

A. Cannot export default and named exports together in one file.

B. Change default export to named export for value and export name as default.

C. Use export default { value, name }; to export both.

D. Add export const name = 'Answer'; alongside default export.

Solution

1. Step 1: Understand export combinations

   JavaScript allows one default export and multiple named exports in the same file.

2. Step 2: Add named export alongside default

   Adding export const name = 'Answer'; exports a named constant while keeping default export.

3. Final Answer:

   Add export const name = 'Answer'; alongside default export. -> Option D

4. Quick Check:

   Default + named exports can coexist [OK]

Hint: Default export plus named exports can be in same file [OK]

Common Mistakes:

• Thinking default and named exports cannot coexist
• Trying to export multiple defaults
• Using object as default export to hold multiple values