Augmenting third-party libraries in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a new property to the existing interface.

Typescript

declare module 'some-library' {
  interface User {
    [1]: string;
  }
}

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

Aemail

Bnickname

Cage

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using a property name that already exists in the interface.

Forgetting to use 'declare module' syntax.

We add the 'nickname' property to the User interface to augment it.

2fill in blank

medium

Complete the code to augment the global Window interface with a new method.

Typescript

declare global {
  interface Window {
    [1](): void;
  }
}

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

AalertUser

BshowUser

CprintUser

DlogUser

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a method with parameters when none are expected.

Not using 'declare global' to augment global interfaces.

We add the 'alertUser' method to the Window interface to augment it.

3fill in blank

hard

Fix the error in the module augmentation by completing the missing keyword.

Typescript

import 'some-library';

declare [1] 'some-library' {
  interface Config {
    debugMode: boolean;
  }
}

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

Ainterface

Bnamespace

Cclass

Dmodule

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'namespace' instead of 'module' for module augmentation.

Omitting the keyword entirely.

The 'module' keyword is required to augment an existing module.

4fill in blank

hard

Fill both blanks to correctly augment the Express Request interface with a new property.

Typescript

import 'express';

declare [1] 'express' {
  interface Request {
    [2]: string;
  }
}

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

Amodule

Bnamespace

CuserId

DsessionId

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'namespace' instead of 'module' for augmentation.

Choosing a property name unrelated to sessions.

Use 'module' to augment 'express' and add 'sessionId' property to Request interface.

5fill in blank

hard

Fill all three blanks to augment the React module by adding a new prop to the ButtonProps interface.

Typescript

import 'react';

declare [1] 'react' {
  interface ButtonProps {
    [2]?: boolean;
  }
  interface ButtonProps {
    [3]?: string;
  }
}

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

Amodule

Bdisabled

Cvariant

Dnamespace

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'namespace' instead of 'module'.

Choosing invalid or unrelated prop names.

Use 'module' to augment 'react' and add 'disabled' and 'variant' props to ButtonProps.

Practice

1. What is the main purpose of augmenting third-party libraries in TypeScript?

easy

A. To add or fix types without changing the original library code

B. To rewrite the entire library code for better performance

C. To remove unused functions from the library

D. To convert JavaScript code to TypeScript automatically

Solution

1. Step 1: Understand the goal of augmentation

   Augmentation is used to add or fix types in existing third-party libraries without modifying their source code.

2. Step 2: Compare options with this goal

   Only To add or fix types without changing the original library code matches this purpose. Other options describe different tasks unrelated to augmentation.

3. Final Answer:

   To add or fix types without changing the original library code -> Option A

4. Quick Check:

   Augmenting = add/fix types without code change [OK]

Hint: Augmenting means adding types, not changing code [OK]

Common Mistakes:

• Thinking augmentation rewrites library code
• Confusing augmentation with code conversion
• Assuming augmentation removes functions

2. Which syntax correctly starts a module augmentation for a library named my-lib?

easy

A. extend module 'my-lib' { }

B. module 'my-lib' { }

C. augment module 'my-lib' { }

D. declare module 'my-lib' { }

Solution

1. Step 1: Recall correct augmentation syntax

   TypeScript uses declare module 'library-name' { } to augment modules.

2. Step 2: Check options for this syntax

   Only declare module 'my-lib' { } uses declare module correctly. Others use invalid keywords.

3. Final Answer:

   declare module 'my-lib' { } -> Option D

4. Quick Check:

   Use declare module for augmentation [OK]

Hint: Always start with declare module 'library-name' [OK]

Common Mistakes:

• Omitting the declare keyword
• Using wrong keywords like augment or extend
• Using module without declare

3. What will be the output of this TypeScript code after augmenting the lodash library to add a new method double to its LoDashStatic interface?

declare module 'lodash' {
  interface LoDashStatic {
    double(n: number): number;
  }
}

import _ from 'lodash';

console.log(_.double(4));

medium

A. Runtime error: _.double is not a function

B. 8

C. TypeScript error: Property 'double' does not exist on type 'LoDashStatic'

D. 4

Solution

1. Step 1: Understand augmentation effect

   Augmentation adds type info but does not add actual implementation code.

2. Step 2: Analyze runtime behavior

   Since _.double is not implemented, calling it causes a runtime error.

3. Final Answer:

   Runtime error: _.double is not a function -> Option A

4. Quick Check:

   Augmentation adds types only, no code [OK]

Hint: Augmentation adds types, not code--runtime error if method missing [OK]

Common Mistakes:

• Assuming augmentation adds implementation automatically
• Expecting TypeScript error instead of runtime error
• Confusing type addition with code addition

4. Identify the error in this module augmentation code for express that tries to add a new property userId to the Request interface:

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

// Usage
import express from 'express';
const app = express();
app.use((req, res, next) => {
  console.log(req.userId);
  next();
});

medium

A. The import statement should use require instead of import

B. The property userId must be optional with a question mark

C. The module name should be 'express-serve-static-core' instead of 'express'

D. The interface Request cannot be augmented

Solution

1. Step 1: Check correct module for Request interface

   In Express, the Request interface is declared in 'express-serve-static-core', not directly in 'express'.

2. Step 2: Understand augmentation target

   To augment Request, you must declare module 'express-serve-static-core', or augmentation won't apply correctly.

3. Final Answer:

   The module name should be 'express-serve-static-core' instead of 'express' -> Option C

4. Quick Check:

   Augment correct module name for interface [OK]

Hint: Augment the exact module where interface is declared [OK]

Common Mistakes:

• Augmenting wrong module name
• Making property optional unnecessarily
• Thinking interface cannot be augmented

5. You want to augment the react-router-dom library to add a new optional property meta of type { requiresAuth: boolean } to the RouteProps interface. Which code snippet correctly achieves this and ensures TypeScript recognizes the new property?

hard

A. interface RouteProps { meta?: { requiresAuth: boolean }; }

// No module declaration needed

B. declare module 'react-router-dom' { interface RouteProps { meta?: { requiresAuth: boolean }; } }

// Then use

C. declare module 'react-router' { interface RouteProps { meta?: { requiresAuth: boolean }; } }

// Wrong module name

D. declare global { interface RouteProps { meta?: { requiresAuth: boolean }; } }

// Incorrect global augmentation

Solution

1. Step 1: Identify correct module to augment

   The RouteProps interface is declared in 'react-router-dom', so we must augment that module.

2. Step 2: Write augmentation with optional property

   Use declare module 'react-router-dom' and add meta?: { requiresAuth: boolean } inside RouteProps.

3. Final Answer:

   declare module 'react-router-dom' { interface RouteProps { meta?: { requiresAuth: boolean }; } } -> Option B

4. Quick Check:

   Augment correct module with optional property [OK]

Hint: Augment exact module and interface with declare module [OK]

Common Mistakes:

• Augmenting wrong module name
• Not using declare module syntax
• Adding property outside interface