ES Modules import and export in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

What is the output of this ES module import code?

Consider two files: mathUtils.mjs and app.mjs.

mathUtils.mjs exports a named function square.
In app.mjs, we import square and call it with 4.

What will console.log(result) print?

Node.js

/* mathUtils.mjs */
export function square(x) {
  return x * x;
}

/* app.mjs */
import { square } from './mathUtils.mjs';
const result = square(4);
console.log(result);

A undefined

B 16

C NaN

D Error: Cannot find module './mathUtils.mjs'

💡 Hint

Remember that named exports must be imported with curly braces matching the exported name.

The function square is exported as a named export and imported correctly. Calling square(4) returns 16, which is logged.

❓ component_behavior

intermediate

What happens when importing a default export incorrectly?

Given utils.mjs exports a default function:

export default function greet() { return 'Hello'; }

Which import statement in app.mjs will cause a runtime error when calling greet()?

A import { greet } from './utils.mjs'; console.log(greet());

B import greet from './utils.mjs'; console.log(greet());

C import * as greet from './utils.mjs'; console.log(greet());

D import { default as greet } from './utils.mjs'; console.log(greet());

💡 Hint

Default exports are imported without curly braces.

Option A tries to import a named export 'greet' which does not exist, causing greet to be undefined and calling greet() throws a TypeError.

🔧 Debug

advanced

Why does this ES module import cause a SyntaxError?

Look at this code snippet in app.mjs:

import { add } from './math.mjs'

Running this code causes a SyntaxError. What is the most likely cause?

Node.js

import { add } from './math.mjs';

A Running the file without the --experimental-modules flag in Node.js 12

B Missing semicolon at the end of the import statement

C Import statements must be at the bottom of the file

D Using single quotes instead of double quotes in import path

💡 Hint

Check Node.js version and module support.

Node.js versions before 13 require the --experimental-modules flag to run ES modules. Without it, import statements cause SyntaxError.

❓ state_output

advanced

What is the value of variable 'count' after these imports and mutations?

In counter.mjs we have:

export let count = 0;
export function increment() { count++; }

In app.mjs:

import { count, increment } from './counter.mjs';
increment();
console.log(count);

What will be logged?

A Error: Assignment to constant variable

B undefined

C 0

D 1

💡 Hint

ES module imports are live bindings: mutations to the exported variable from within the module are visible to importers.

Imports in ES modules are live bindings to the exports. The increment() function mutates the shared 'count' variable, so the imported binding sees the updated value of 1.

🧠 Conceptual

expert

Which option correctly re-exports a named export with a new name?

You want to re-export a named export foo from moduleA.mjs as bar in moduleB.mjs.

Which code in moduleB.mjs achieves this?

A export * as bar from './moduleA.mjs';

B import { foo as bar } from './moduleA.mjs'; export { foo };

C export { foo as bar } from './moduleA.mjs';

D export { bar } from './moduleA.mjs';

💡 Hint

Use export with alias syntax.

Option C correctly re-exports 'foo' as 'bar'. Option C imports with alias but exports original name 'foo' which is undefined here. Option C exports all as 'bar' namespace, not a renamed export. Option C tries to export 'bar' which does not exist in moduleA.