Declaration file syntax (.d.ts) in Typescript - Mini Project: Build & Apply

Understanding Declaration File Syntax (.d.ts) in TypeScript

📖 Scenario: You are working on a TypeScript project that uses a JavaScript library without type definitions. To help TypeScript understand the library's types, you will create a declaration file (.d.ts).

🎯 Goal: Create a declaration file that describes a simple JavaScript library's interface using TypeScript declaration syntax.

📋 What You'll Learn

Create an interface describing a user object

Declare a function that takes a user object and returns a string

Use the declare keyword properly

Export the function from the declaration file

💡 Why This Matters

🌍 Real World

Declaration files help TypeScript understand JavaScript libraries without built-in types, improving code safety and editor support.

💼 Career

Many jobs require working with third-party JavaScript libraries in TypeScript projects, so knowing how to write declaration files is valuable.

1

Create an interface for a user object

Create an interface called User with two properties: name of type string and age of type number.

Typescript

# Your code here
// Create interface User with name and age properties

Hint

Use the interface keyword followed by the interface name and curly braces to define properties.

2

Declare a function that uses the User interface

Add a declaration for a function called greetUser that takes one parameter user of type User and returns a string. Use the declare keyword.

Typescript

interface User {
    name: string;
    age: number;
}

// Declare function greetUser that takes user: User and returns string
# Your code here

Hint

Use declare function followed by the function name and parameter types.

3

Export the declared function

Export the greetUser function using the export keyword so it can be used by other files.

Typescript

interface User {
    name: string;
    age: number;
}

declare function greetUser(user: User): string;

// Export greetUser function
# Your code here

Hint

Use export { functionName }; syntax to export declarations.

4

Print a message showing the declaration file content

Print the entire declaration file content as a single string exactly as written, including interface, function declaration, and export statement.

Typescript

interface User {
    name: string;
    age: number;
}

declare function greetUser(user: User): string;

export { greetUser };

// Print the declaration file content
# Your code here

Hint

Use console.log with a template string to print the full content.

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