Declaring modules in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a module named "myModule".

Typescript

declare module [1] {
  export const greeting: string;
}

Drag options to blanks, or click blank then click option'

A'myModule'

B"myModule"

CmyModule

D`myModule`

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to use quotes around the module name.

Using backticks or no quotes which causes syntax errors.

The module name must be a string literal enclosed in quotes. Using double quotes is standard.

2fill in blank

medium

Complete the code to export a function named "sayHello" inside the module.

Typescript

declare module "utils" {
  export function [1](): void;
}

Drag options to blanks, or click blank then click option'

AsayHello

Bhello

Cgreet

DsayHi

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different function name than declared.

Misspelling the function name.

The function name must match the export name "sayHello" as declared.

3fill in blank

hard

Fix the error in the module declaration by completing the code correctly.

Typescript

declare module [1] {
  export interface User {
    id: number;
    name: string;
  }
}

Drag options to blanks, or click blank then click option'

A"UserModule"

B'UserModule'

CUserModule

D`UserModule`

Attempts:

3 left

💡 Hint

Common Mistakes

Using single quotes or backticks instead of double quotes.

Not using quotes at all.

The module name must be a string literal with double quotes to be valid.

4fill in blank

hard

Fill both blanks to declare a module "mathLib" that exports a constant PI with value 3.14.

Typescript

declare module [1] {
  export const PI: [2];
}

Drag options to blanks, or click blank then click option'

A"mathLib"

Bnumber

Cstring

D"PI"

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong type for PI like string.

Not quoting the module name.

The module name must be a string literal "mathLib" and the type of PI is number.

5fill in blank

hard

Fill all three blanks to declare a module "shapes" exporting a type Circle and a function area.

Typescript

declare module [1] {
  export type [2] = { radius: number };
  export function [3](c: Circle): number;
}

Drag options to blanks, or click blank then click option'

A"shapes"

BCircle

Carea

DShape

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect module name without quotes.

Mismatching type or function names.

The module name is "shapes", the type is Circle, and the function is named area.

Practice

1. What is the main purpose of using declare module in TypeScript?

easy

A. To import modules dynamically at runtime

B. To create new modules with implementation

C. To execute JavaScript code inside TypeScript files

D. To tell TypeScript about the shape of a module without actual code

Solution

1. Step 1: Understand the role of declare module

   This keyword is used to describe the shape or types of a module when TypeScript has no information about it.

2. Step 2: Differentiate from other module actions

   It does not import, execute, or create code but only declares types for existing modules.

3. Final Answer:

   To tell TypeScript about the shape of a module without actual code -> Option D

4. Quick Check:

   declare module = describe types only [OK]

Hint: Declare modules to describe types, not to run code [OK]

Common Mistakes:

• Thinking declare module imports or runs code
• Confusing declare module with creating actual modules
• Using declare module to execute JavaScript

2. Which of the following is the correct syntax to declare a module named "my-lib" in TypeScript?

easy

A. declare module 'my-lib' { export function foo(): void; }

B. module declare 'my-lib' { export function foo(): void; }

C. declare 'my-lib' module { export function foo(): void; }

D. module 'my-lib' declare { export function foo(): void; }

Solution

1. Step 1: Recall correct syntax for module declaration

   The correct syntax starts with declare module 'module-name' followed by curly braces containing declarations.

2. Step 2: Check each option's order and keywords

   Only declare module 'my-lib' { export function foo(): void; } uses the correct order: declare module 'my-lib' { ... }. Others mix keywords incorrectly.

3. Final Answer:

   declare module 'my-lib' { export function foo(): void; } -> Option A

4. Quick Check:

   declare module 'name' { ... } syntax [OK]

Hint: Start with 'declare module' then module name in quotes [OK]

Common Mistakes:

• Swapping 'module' and 'declare' keywords
• Omitting quotes around module name
• Using wrong keyword order

3. Given this declaration file:

declare module 'math-utils' {
  export function add(a: number, b: number): number;
}

What will be the output of this TypeScript code?

import { add } from 'math-utils';
console.log(add(2, 3));

medium

A. undefined

B. Runtime error: add is not a function

C. Compile error: Cannot find module 'math-utils'

D. 5

Solution

1. Step 1: Understand what declare module does

   It only tells TypeScript about types but does not provide actual implementation.

2. Step 2: Analyze runtime behavior

   Since no real code exists for 'math-utils', importing and calling add will cause a runtime error because add is undefined.

3. Final Answer:

   Runtime error: add is not a function -> Option B

4. Quick Check:

   declare module = types only, no code [OK]

Hint: Declare modules don't provide code, so runtime fails [OK]

Common Mistakes:

• Assuming declare module provides implementation
• Expecting correct output without real code
• Confusing compile-time types with runtime code

4. Identify the error in this module declaration:

declare module myLib {
  export const version: string;
}

medium

A. Missing function declaration

B. Cannot export constants in declare module

C. Module name must be in quotes

D. Syntax is correct, no error

Solution

1. Step 1: Check module name syntax

   Module names must be string literals in quotes, like 'myLib', not bare identifiers.

2. Step 2: Verify export syntax

   Exporting constants is allowed, so no error there.

3. Final Answer:

   Module name must be in quotes -> Option C

4. Quick Check:

   Module names need quotes [OK]

Hint: Always put module names in quotes in declare module [OK]

Common Mistakes:

• Omitting quotes around module name
• Thinking export const is invalid
• Assuming function declaration is mandatory

5. You want to declare a module "config" that exports an object with optional properties apiUrl (string) and timeout (number). Which declaration correctly models this?

hard

A. declare module 'config' {
export const settings: { apiUrl?: string; timeout?: number };
}

B. declare module 'config' {
export const settings: { apiUrl: string; timeout: number };
}

C. declare module 'config' {
export function settings(): { apiUrl?: string; timeout?: number };
}

D. declare module 'config' {
export let settings: { apiUrl: string; timeout?: number };
}

Solution

1. Step 1: Understand optional properties syntax

   Optional properties use the ? symbol after the property name.

2. Step 2: Match declaration to requirements

   declare module 'config' { export const settings: { apiUrl?: string; timeout?: number }; } correctly declares settings as an exported constant object with both properties optional.

3. Step 3: Check other options

   declare module 'config' { export const settings: { apiUrl: string; timeout: number }; } requires both properties (not optional). declare module 'config' { export function settings(): { apiUrl?: string; timeout?: number }; } exports a function, not an object. declare module 'config' { export let settings: { apiUrl: string; timeout?: number }; } makes apiUrl required, which is incorrect.

4. Final Answer:

   declare module 'config' { export const settings: { apiUrl?: string; timeout?: number }; } -> Option A

5. Quick Check:

   Optional props use ? in type declarations [OK]

Hint: Use ? after property name for optional fields [OK]

Common Mistakes:

• Forgetting ? for optional properties
• Using function instead of object export
• Making required properties when optional needed