Module augmentation syntax in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to augment the existing module by adding a new function.

Typescript

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

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

AnewFunction

BoldFunction

CmainFunction

DstartFunction

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name that already exists in the module.

Forgetting to use the 'export' keyword.

The new function added to the module is named 'newFunction'.

2fill in blank

medium

Complete the code to augment the module by adding a new interface.

Typescript

declare module "myModule" {
  export interface [1] {
    id: number;
  }
}

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

AUser

BProduct

COrder

DItem

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing an unrelated interface name.

Not using PascalCase for interface names.

The interface 'User' is added to the module to represent a user object.

3fill in blank

hard

Fix the error in the module augmentation syntax by completing the code.

Typescript

declare module "myModule" [1] {
  export const version: string;
}

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

Aextends

B{

Caugment

Ddeclare

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'extends' or 'augment' keywords which are invalid here.

Missing the opening curly brace.

The module block must start with a curly brace '{' after the module name.

4fill in blank

hard

Complete the code to correctly augment the module by adding a new type alias.

Typescript

declare module "myModule" { {
  export type {BLANK_2}} = string | number;
}

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

A{

BAliasType

CAugmentType

D}

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect block delimiters.

Choosing a type alias name that is not descriptive.

The module block opens with '{' and the new type alias is named 'AliasType'.

5fill in blank

hard

Fill both blanks to augment the module by adding a new namespace with a function.

Typescript

declare module "myModule" { 
  export namespace [1] {
    export function [2](): void;
  }
}

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

A{

BUtils

ClogMessage

DHelpers

Attempts:

3 left

💡 Hint

Common Mistakes

Missing the opening brace for the module block.

Using invalid or unrelated names for namespace or function.

The module block opens with '{', the namespace is 'Utils', and the function is 'logMessage'.

Practice

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

easy

A. To create a new module from scratch

B. To delete types from a module

C. To add or extend types in an existing module without modifying its original source code

D. To convert JavaScript code into TypeScript

Solution

1. Step 1: Understand module augmentation purpose

   Module augmentation allows adding or extending types in existing modules without changing their original code.

2. Step 2: Compare options

   Only To add or extend types in an existing module without modifying its original source code correctly describes this purpose. Options A, C, and D describe unrelated actions.

3. Final Answer:

   To add or extend types in an existing module without modifying its original source code -> Option C

4. Quick Check:

   Module augmentation = extend existing types [OK]

Hint: Module augmentation means adding types to existing modules [OK]

Common Mistakes:

• Thinking it creates new modules
• Confusing augmentation with deleting types
• Assuming it converts JS to TS

2. Which of the following is the correct syntax to start a module augmentation for a module named "my-library"?

easy

A. module augment "my-library" { /* augmentation here */ }

B. declare module "my-library" { /* augmentation here */ }

C. augment module "my-library" { /* augmentation here */ }

D. module "my-library" augment { /* augmentation here */ }

Solution

1. Step 1: Recall correct syntax for module augmentation

   The correct syntax uses declare module "module-name" { ... } to augment a module.

2. Step 2: Check options

   Only declare module "my-library" { /* augmentation here */ } matches this syntax exactly. Others use invalid keywords or order.

3. Final Answer:

   declare module "my-library" { /* augmentation here */ } -> Option B

4. Quick Check:

   Use declare module "name" for augmentation [OK]

Hint: Start with 'declare module "module-name"' for augmentation [OK]

Common Mistakes:

• Using wrong keywords like 'augment' or 'module augment'
• Incorrect order of keywords
• Omitting 'declare'

3. Given the following code, what will be the output of console.log(myLib.newFunction());?

import * as myLib from "my-library";

declare module "my-library" {
  export function newFunction(): string;
}

myLib.newFunction = () => "Hello from augmentation!";

console.log(myLib.newFunction());

medium

A. "Hello from augmentation!"

B. TypeError: myLib.newFunction is not a function

C. undefined

D. SyntaxError

Solution

1. Step 1: Understand augmentation effect

   The module augmentation adds a new function type declaration to "my-library". Then the function is assigned an implementation.

2. Step 2: Check runtime behavior

   Calling myLib.newFunction() runs the assigned arrow function returning the string "Hello from augmentation!".

3. Final Answer:

   "Hello from augmentation!" -> Option A

4. Quick Check:

   Augmented function runs and returns string [OK]

Hint: Augmented functions can be assigned and called normally [OK]

Common Mistakes:

• Assuming augmentation adds runtime code automatically
• Expecting TypeError because function was not originally present
• Confusing type declaration with implementation

4. Identify the error in this module augmentation code:

declare module "utils" {
  export interface Config {
    timeout: number;
  }
}

declare module "utils" {
  export interface Config {
    retries: number;
  }
}

const config: Config = { timeout: 1000, retries: 3 };

medium

A. No error, code is correct

B. The interface Config is declared twice without merging

C. The module name "utils" is invalid

D. The variable config is missing import from "utils"

Solution

1. Step 1: Check interface merging

   Interfaces with the same name in module augmentation merge correctly, so no error there.

2. Step 2: Check usage of Config

   The variable config uses Config but does not import or reference it properly, causing an error.

3. Final Answer:

   The variable config is missing import from "utils" -> Option D

4. Quick Check:

   Missing import causes error, not interface duplication [OK]

Hint: Always import types before using them outside modules [OK]

Common Mistakes:

• Thinking interface redeclaration causes error
• Ignoring missing imports
• Assuming module name is invalid

5. You want to add a new method logInfo(message: string): void to the existing Logger class from a third-party module "logger-lib" without modifying its source. Which is the correct way to do this using module augmentation?

hard

A. declare module "logger-lib" {
interface Logger {
logInfo(message: string): void;
}
}
Logger.prototype.logInfo = function(message) { console.log(message); };

B. module "logger-lib" {
class Logger {
logInfo(message: string): void {}
}
}

C. declare module "logger-lib" {
class Logger {
logInfo(message: string): void {}
}
}

D. augment module "logger-lib" {
interface Logger {
logInfo(message: string): void;
}
}

Solution

1. Step 1: Use correct module augmentation syntax

   Use declare module "logger-lib" { interface Logger { ... } } to add method signature.

2. Step 2: Add method implementation to prototype

   Assign the method to Logger.prototype to add runtime behavior without modifying original source.

3. Final Answer:

   declare module "logger-lib" { interface Logger { logInfo(message: string): void; } } Logger.prototype.logInfo = function(message) { console.log(message); }; -> Option A

4. Quick Check:

   Declare interface then add method to prototype [OK]

Hint: Declare interface then add method to prototype for augmentation [OK]

Common Mistakes:

• Using 'module' instead of 'declare module'
• Declaring class instead of interface in augmentation
• Forgetting to add method implementation