Default exports in Javascript - Time & Space Complexity

Time Complexity: Default exports

O(n)

Understanding Time Complexity

When using default exports in JavaScript modules, it's helpful to understand how importing and exporting affect performance.

We want to see how the cost of using default exports grows as the size of the exported content changes.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

// module.js
const bigData = new Array(1000).fill(0).map((_, i) => i);
export default bigData;

// main.js
import data from './module.js';
console.log(data.length);
    

This code exports a large array as a default export and imports it in another file.

Identify Repeating Operations

• Primary operation: Creating and filling the array with 1000 elements.
• How many times: The map function runs 1000 times to fill the array.

How Execution Grows With Input

As the size of the exported array grows, the time to create it grows proportionally.

Input Size (n)	Approx. Operations
10	10 operations to fill array
100	100 operations to fill array
1000	1000 operations to fill array

Pattern observation: The operations increase directly with the size of the exported data.

Final Time Complexity

Time Complexity: O(n)

This means the time to prepare the default export grows linearly with the size of the data.

Common Mistake

[X] Wrong: "Importing a default export is always instant and cost-free."

[OK] Correct: The cost depends on what is being exported; large data structures take time to create and load.

Interview Connect

Understanding how default exports handle data size helps you reason about module loading and performance in real projects.

Self-Check

"What if we changed the default export to a function that returns the data instead? How would the time complexity change?"

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