Module augmentation syntax in Typescript - Mini Project: Build & Apply

Module augmentation syntax

📖 Scenario: You are working on a TypeScript project that uses a third-party module called shapes. You want to add a new function to this module without changing its original code.

🎯 Goal: Learn how to use module augmentation syntax in TypeScript to add a new function getCircleArea to the existing shapes module.

📋 What You'll Learn

Create a module named shapes with an existing function getSquareArea.

Add a module augmentation to shapes that declares a new function getCircleArea.

Implement the new function getCircleArea inside the module augmentation.

Call both getSquareArea and getCircleArea and print their results.

💡 Why This Matters

🌍 Real World

Module augmentation is useful when you want to add features to existing libraries or modules without modifying their original source code.

💼 Career

Many TypeScript projects use third-party libraries. Knowing how to safely extend these libraries with module augmentation is a valuable skill for professional developers.

1

Create the initial shapes module

Create a module named shapes that exports a function called getSquareArea. This function takes a number side and returns the area of a square (side * side).

Typescript

// Your code here

Hint

Use export module shapes and inside it define export function getSquareArea(side: number): number.

2

Add module augmentation declaration

Add a module augmentation for the shapes module that declares a new function called getCircleArea which takes a number radius and returns a number.

Typescript

export module shapes {
    export function getSquareArea(side: number): number {
        return side * side;
    }
}

// Your code here

Hint

Use declare module "shapes" and inside it declare export function getCircleArea(radius: number): number;.

3

Implement the new function inside module augmentation

Implement the getCircleArea function inside the shapes module augmentation. It should return the area of a circle using the formula Math.PI * radius * radius.

Typescript

export module shapes {
    export function getSquareArea(side: number): number {
        return side * side;
    }
}

declare module "shapes" {
    export function getCircleArea(radius: number): number;
}

// Implement getCircleArea function here

Hint

Inside export module shapes, add export function getCircleArea(radius: number): number with the correct return statement.

4

Call and print results of both functions

Call shapes.getSquareArea(4) and shapes.getCircleArea(3). Print their results using console.log.

Typescript

export module shapes {
    export function getSquareArea(side: number): number {
        return side * side;
    }

    export function getCircleArea(radius: number): number {
        return Math.PI * radius * radius;
    }
}

declare module "shapes" {
    export function getCircleArea(radius: number): number;
}

// Call and print results here

Hint

Use console.log(shapes.getSquareArea(4)) and console.log(shapes.getCircleArea(3)) to print the results.

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