Declaration file syntax (.d.ts) in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this declaration usage?

Given the following declaration file and usage, what will be the output when running the compiled JavaScript?

Typescript

declare module "mathUtils" {
  export function add(a: number, b: number): number;
}

import { add } from "mathUtils";
console.log(add(3, 4));

A undefined

B 7

C TypeError: add is not a function

D Compilation error

💡 Hint

Declaration files only describe types and do not provide implementations.

The declaration file .d.ts only tells TypeScript about the shape of the module but does not provide any actual code. When running the compiled JavaScript, since there is no implementation for add, it results in a runtime TypeError.

🧠 Conceptual

intermediate

Which syntax correctly declares a constant in a .d.ts file?

Select the correct way to declare a constant named PI with type number in a declaration file.

A declare const PI: number;

B const PI: number = 3.14;

C declare let PI: number = 3.14;

D export const PI = 3.14;

💡 Hint

Declaration files only declare types and do not assign values.

In declaration files, you use declare to tell TypeScript about the existence and type of variables without assigning values. Option A correctly declares PI as a constant of type number without initialization.

🔧 Debug

advanced

Identify the syntax error in this .d.ts interface declaration

Find the syntax error in the following declaration file snippet:

Typescript

interface User {
  name: string;
  age: number;
  greet(): void
}

A Missing semicolon after 'name: string'

B Interface keyword should be 'declare interface'

C Methods cannot be declared without implementation in .d.ts files

D Property 'age' should be optional

💡 Hint

Check punctuation carefully in interface declarations.

In TypeScript interfaces, each property declaration must end with a semicolon or comma. The line name: string is missing a semicolon, causing a syntax error.

🚀 Application

advanced

How to declare a function with overloads in a .d.ts file?

You want to declare a function format that can be called with either one string argument or two string arguments. Which declaration is correct in a .d.ts file?

A declare function format(text: string | (text: string, style: string)): string;

B

declare function format(text: string): string;
declare function format(text: string, style: string): string;

C declare function format(text: string, style?: string): string;

D declare function format(...args: string[]): string;

💡 Hint

Overloads are declared as multiple function declarations with the same name.

Function overloads in declaration files are declared as multiple declare function statements with the same name but different parameters. Option B correctly declares two overloads for format.

❓ Predict Output

expert

What is the type of 'data' after importing from this declaration file?

Given this declaration file snippet and usage, what is the type of data in the TypeScript code?

Typescript

declare module "dataModule" {
  interface Data {
    id: number;
    value: string;
  }
  const data: Data[];
  export default data;
}

import data from "dataModule";

// What is the type of 'data' here?

A Object with properties 'id' and 'value' but not an array

B Any

C Unknown

D Array of objects with properties 'id' (number) and 'value' (string)

💡 Hint

Look at the declared type of 'data' in the module.

The declaration states const data: Data[]; which means data is an array of Data objects. Each Data object has id as a number and value as a string.

Practice

1. What is the main purpose of a TypeScript declaration file (.d.ts)?

easy

A. To describe the types and structure of existing JavaScript code without implementation

B. To write executable TypeScript code

C. To compile TypeScript into JavaScript

D. To store runtime configuration settings

Solution

1. Step 1: Understand the role of declaration files

   Declaration files provide type information about existing JavaScript code but do not contain actual code implementations.

2. Step 2: Differentiate from other file types

   Unlike .ts files that contain executable code, .d.ts files only describe types and interfaces.

3. Final Answer:

   To describe the types and structure of existing JavaScript code without implementation -> Option A

4. Quick Check:

   Declaration files describe types only [OK]

Hint: Declaration files describe types, not code execution [OK]

Common Mistakes:

• Thinking declaration files contain executable code
• Confusing declaration files with config files
• Assuming declaration files compile to JavaScript

2. Which of the following is the correct syntax to declare a function named greet that takes a string and returns void in a .d.ts file?

easy

A. function greet(name: string): void;

B. function greet(name: string): void {}

C. declare greet(name: string): void;

D. declare function greet(name: string): void;

Solution

1. Step 1: Recognize declaration syntax in .d.ts files

   Functions must be declared with the declare keyword and no implementation body.

2. Step 2: Check each option

   function greet(name: string): void; misses declare, declare greet(name: string): void; has incorrect syntax, function greet(name: string): void {} includes a function body which is not allowed in declaration files.

3. Final Answer:

   declare function greet(name: string): void; -> Option D

4. Quick Check:

   Use declare function without body [OK]

Hint: Use 'declare function' with no body in .d.ts files [OK]

Common Mistakes:

• Omitting the 'declare' keyword
• Adding function bodies in declaration files
• Using incorrect declaration syntax

3. Given the following declaration in a .d.ts file:

declare module "math-lib" {
  export function add(a: number, b: number): number;
}

What will be the TypeScript type of result after this code?

import { add } from "math-lib";
const result = add(5, 3);

medium

A. string

B. void

C. number

D. any

Solution

1. Step 1: Analyze the declaration file

   The module "math-lib" declares an exported function add that takes two numbers and returns a number.

2. Step 2: Understand the import and usage

   The code imports add and calls it with two numbers, so the result type matches the declared return type: number.

3. Final Answer:

   number -> Option C

4. Quick Check:

   Declared return type = number [OK]

Hint: Return type in declaration matches usage type [OK]

Common Mistakes:

• Assuming return type is any by default
• Confusing return type with parameter types
• Ignoring module declaration types

4. Identify the error in this .d.ts declaration:

declare var count: number = 10;

medium

A. Variables cannot be declared with initial values in .d.ts files

B. The keyword 'var' is not allowed in declaration files

C. The type 'number' is invalid in declaration files

D. The 'declare' keyword is missing

Solution

1. Step 1: Understand variable declaration in .d.ts files

   Declaration files only declare types and variables without assigning values.

2. Step 2: Check the given code

   The code assigns = 10 which is not allowed in declaration files.

3. Final Answer:

   Variables cannot be declared with initial values in .d.ts files -> Option A

4. Quick Check:

   No initial values allowed in declarations [OK]

Hint: No initial values in declarations, only types [OK]

Common Mistakes:

• Adding initial values to declared variables
• Removing 'declare' keyword mistakenly
• Using invalid keywords in declaration files

5. You want to write a declaration file for a JavaScript library that exports an object with keys as strings and values as functions returning numbers. Which is the correct declaration syntax in a .d.ts file?

hard

A. declare const lib: Map number>; export default lib;

B. declare const lib: { [key: string]: () => number }; export default lib;

C. declare const lib: { key: string; value: () => number }; export default lib;

D. const lib: { string: () => number }; export default lib;

Solution

1. Step 1: Understand the object shape needed

   The library exports an object where keys are strings and values are functions returning numbers.

2. Step 2: Match syntax for index signatures in declaration files

   declare const lib: { [key: string]: () => number }; export default lib; uses the correct index signature syntax { [key: string]: () => number } with declare and exports default.

3. Step 3: Eliminate incorrect options

   const lib: { string: () => number }; export default lib; lacks declare and uses invalid syntax, declare const lib: { key: string; value: () => number }; export default lib; incorrectly declares keys and values as properties, declare const lib: Map number>; export default lib; uses Map which is not specified.

4. Final Answer:

   declare const lib: { [key: string]: () => number }; export default lib; -> Option B

5. Quick Check:

   Use index signature with declare and export default [OK]

Hint: Use index signature {[key: string]: type} for dynamic keys [OK]

Common Mistakes:

• Using invalid object property syntax
• Omitting 'declare' keyword
• Confusing Map with plain object types