Module augmentation syntax in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of module augmentation with interface merging

What is the output of this TypeScript code when compiled and run with Node.js (using ts-node or similar)?

Typescript

declare module "my-module" {
  interface User {
    name: string;
  }
}

// Augmenting the module
import "my-module";
declare module "my-module" {
  interface User {
    age: number;
  }
}

const user: import("my-module").User = { name: "Alice", age: 30 };
console.log(user);

A TypeError: Cannot read property 'name' of undefined

B {"name":"Alice","age":30}

C SyntaxError: Duplicate identifier 'User'

D {"name":"Alice"}

💡 Hint

Think about how TypeScript merges interfaces in module augmentation.

TypeScript merges interface declarations with the same name inside module augmentation. The User interface will have both 'name' and 'age' properties. The object matches the merged interface, so the output is the full object with both properties.

🧠 Conceptual

intermediate

Purpose of module augmentation syntax

What is the main purpose of using module augmentation syntax in TypeScript?

A To add new exports or extend existing types in an external module without modifying its original source code

B To import all exports from a module under a new namespace

C To create a new module with the same name as an existing one, replacing it completely

D To declare global variables accessible in all modules

💡 Hint

Think about extending functionality without changing original files.

Module augmentation allows you to add new types or members to an existing module's declarations, useful for extending third-party modules safely.

🔧 Debug

advanced

Identify the error in this module augmentation

This code tries to augment a module but causes a compilation error. What is the error?

Typescript

declare module "library" {
  export function greet(): void;
}

declare module "library" {
  export const version: string;
}

// Usage
import { greet, version } from "library";
greet();
console.log(version);

A Cannot redeclare exported function 'greet' without implementation

B Module 'library' has no exported member 'version'

C Duplicate identifier 'library'

D No error, code compiles and runs fine

💡 Hint

Check if the augmentation declares implementations or only types.

Module augmentation can only add types or declarations, but cannot redeclare exported functions without implementation. The first declaration declares a function without implementation, causing an error.

📝 Syntax

advanced

Correct syntax for module augmentation

Which option shows the correct syntax to augment the 'express' module by adding a new property 'userId' of type string to the Request interface?

A

declare module express {
  interface Request {
    userId: string;
  }
}

B

module "express" {
  interface Request {
    userId: string;
  }
}

C

augment module "express" {
  interface Request {
    userId: string;
  }
}

D

declare module "express" {
  interface Request {
    userId: string;
  }
}

💡 Hint

Remember the keyword to declare module augmentation.

The correct syntax uses 'declare module "module-name"' with quotes around the module name. Other options miss 'declare' or quotes or use invalid keywords.

🚀 Application

expert

Resulting type after multiple module augmentations

Given these two separate module augmentation files for module 'config', what is the type of Config after both are loaded?

Typescript

// File 1
declare module "config" {
  interface Config {
    host: string;
  }
}

// File 2
declare module "config" {
  interface Config {
    port: number;
  }
}

// Usage
import type { Config } from "config";
const serverConfig: Config = { host: "localhost", port: 8080 };

A Config has only the last declared property: { port: number }

B Config has only the first declared property: { host: string }

C Config has both properties: { host: string; port: number }

D Config is an empty interface with no properties

💡 Hint

Think about how TypeScript merges interfaces with the same name in module augmentation.

TypeScript merges interface declarations with the same name inside the same module, so Config will have both 'host' and 'port' properties.

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