Default exports in Javascript - Step-by-Step Execution

Concept Flow - Default exports

Create module file

↓

Write export default statement

↓

Import module in another file

↓

Use imported default export

↓

Run program

This flow shows how a JavaScript module defines a default export, which is then imported and used in another file.

Execution Sample

Javascript

export default function greet() {
  return "Hello!";
}

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

This code exports a default function greet and imports it to print a greeting.

Execution Table

Step	Action	Evaluation	Result
1	Define default export function greet	Function greet created	greet is ready to export
2	Export greet as default	Module exports greet as default	Default export set
3	Import default export as greet	greet refers to default export	greet function imported
4	Call greet()	Execute greet function	"Hello!" returned
5	console.log output	Print returned value	Hello! printed to console

💡 Program ends after printing the greeting.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
greet	undefined	function greet()	function greet()	function greet()

Key Moments - 3 Insights

Why do we not use curly braces when importing a default export?

Because the execution_table row 3 shows the import uses the syntax without braces to get the default export directly.

Can a module have more than one default export?

No, as shown in step 2, only one default export is allowed per module.

What happens if you import a default export with a different name?

The imported name is just a local alias, so you can name it anything, as shown in step 3 where 'greet' is the local name.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 5?

A undefined printed

B Error thrown

C "Hello!" printed to console

D Nothing printed

💡 Hint

Check the 'Result' column in step 5 of the execution_table.

At which step is the default export set in the module?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column describing export default in the execution_table.

If we change the import name from 'greet' to 'hello', what changes in the variable_tracker?

A The variable name changes to 'hello' but value stays the same

B The function changes behavior

C The import fails

D No change at all

💡 Hint

Variable names in variable_tracker reflect local names after import, see step 3.

Concept Snapshot

Default exports allow a module to export a single value or function.
Use 'export default' to set it.
Import without braces to get the default export.
You can name the import anything you want.
Only one default export per module.

Full Transcript

This visual execution trace shows how JavaScript default exports work. First, a function greet is defined and exported as the default export of a module. Then, in another file, this default export is imported using the syntax without curly braces and assigned to the local name greet. Calling greet() runs the function and returns 'Hello!'. Finally, console.log prints this returned string. The variable tracker shows greet as a function throughout. Key points include that default exports are imported without braces, only one default export is allowed, and the import name is a local alias. The quizzes test understanding of output, export step, and renaming imports.

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