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?"