Declaring modules in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a module in TypeScript?

A module is a file that contains code (variables, functions, classes) and can export or import them to share with other files.

beginner

How do you declare a module in TypeScript?

You declare a module by creating a file and using export to share parts of the code, and import to use code from other modules.

intermediate

What is the difference between a global script and a module in TypeScript?

A global script shares variables globally without imports/exports. A module has its own scope and shares code only through exports and imports.

intermediate

What does the declare module syntax do in TypeScript?

It tells TypeScript about the shape of a module that exists elsewhere, often used to describe modules without TypeScript code (like JavaScript libraries).

beginner

How do you export a function named greet from a module?

Use <code>export function greet() { ... }</code> to make the function available to other modules.

Which keyword is used to share code from a TypeScript module?

A export

B share

C include

D module

The export keyword is used to share code from a module.

How do you use code from another module in TypeScript?

A import

B require

C include

D use

The import keyword is used to bring code from another module.

What does declare module 'xyz' do?

A Runs the module 'xyz'

B Imports the module named 'xyz'

C Defines the shape of an external module for TypeScript

D Exports all code from module 'xyz'

It declares the type information for an external module.

Which of these is true about modules in TypeScript?

A Modules are the same as classes

B Modules share variables globally by default

C Modules cannot export functions

D Modules have their own scope

Modules have their own scope and share code only via exports.

What file extension is typically used for a TypeScript module?

A .js

B .ts

C .json

D .tsx

TypeScript modules usually use the .ts extension.

Explain how to declare and use a module in TypeScript.

Think about how you share and use code between files.

Describe the purpose of the declare module statement in TypeScript.

It helps TypeScript know about modules without code.

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