Writing custom declaration files in Typescript - Mini Project: Build & Apply

Writing custom declaration files

📖 Scenario: You are using a JavaScript library called mathTools in your TypeScript project. This library does not have built-in TypeScript support, so you want to create a custom declaration file to help TypeScript understand the library's functions and types.

🎯 Goal: Create a custom declaration file mathTools.d.ts that describes the add and multiply functions of the mathTools library, then use these functions in a TypeScript file with proper type checking.

📋 What You'll Learn

Create a declaration file named mathTools.d.ts with the correct module and function declarations.

Declare two functions: add and multiply, each accepting two numbers and returning a number.

Import and use these functions in a TypeScript file named app.ts.

Print the results of calling add(5, 3) and multiply(4, 7).

💡 Why This Matters

🌍 Real World

Many JavaScript libraries do not come with TypeScript types. Writing custom declaration files helps you use these libraries safely in TypeScript projects.

💼 Career

Understanding how to write declaration files is important for TypeScript developers working with third-party JavaScript code or legacy libraries without type support.

1

Create the declaration file with module and function signatures

Create a file named mathTools.d.ts and declare a module called "mathTools". Inside it, declare two functions: add and multiply. Both functions should accept two parameters named a and b of type number and return a number.

Typescript

// Your code here

Hint

Use declare module "mathTools" { ... } to describe the external library module. Inside, declare the functions with their parameter and return types.

2

Import the functions from the custom declaration file

In a new file named app.ts, import the add and multiply functions from the "mathTools" module using ES module syntax.

Typescript

// Your code here

Hint

Use import { add, multiply } from "mathTools"; to import the declared functions.

3

Use the imported functions to calculate values

In app.ts, create two variables: sum that stores the result of add(5, 3) and product that stores the result of multiply(4, 7).

Typescript

// Your code here

Hint

Call the imported functions with the specified arguments and assign the results to variables.

4

Print the results to the console

Add two console.log statements in app.ts to print the values of sum and product.

Typescript

// Your code here

Hint

Use console.log(sum); and console.log(product); to display the results.

Practice

1. What is the main purpose of writing a custom declaration file in TypeScript?

easy

A. To tell TypeScript about types in JavaScript or untyped libraries

B. To write JavaScript code that runs faster

C. To convert TypeScript code into JavaScript

D. To create new JavaScript functions

Solution

1. Step 1: Understand the role of declaration files

   Declaration files provide type information about JavaScript code or libraries without types.

2. Step 2: Identify the purpose in TypeScript

   This helps TypeScript check code and improve editor support by knowing types.

3. Final Answer:

   To tell TypeScript about types in JavaScript or untyped libraries -> Option A

4. Quick Check:

   Custom declaration files = type info for JS [OK]

Hint: Declaration files describe types for untyped JS code [OK]

Common Mistakes:

• Thinking declaration files speed up code execution
• Confusing declaration files with code transpilation
• Believing declaration files create new functions

2. Which syntax correctly declares a module named 'my-lib' in a custom declaration file?

easy

A. module declare 'my-lib' { function foo(): void; }

B. declare module '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 module declaration syntax

   TypeScript uses declare module 'name' { ... } to declare modules.

2. Step 2: Check each option's syntax

   Only declare module 'my-lib' { export function foo(): void; } uses the correct order and keywords.

3. Final Answer:

   declare module 'my-lib' { export function foo(): void; } -> Option B

4. Quick Check:

   Correct module syntax = declare module 'name' [OK]

Hint: Use 'declare module' followed by module name in quotes [OK]

Common Mistakes:

• Swapping 'declare' and 'module' keywords
• Omitting quotes around module name
• Using incorrect keyword order

3. Given this declaration file snippet:

declare module 'utils' {
  export function greet(name: string): string;
}

What is the correct TypeScript output for greet('Alice') if implemented as return `Hello, ${name}!`;?

medium

A. "Hello, Alice!"

B. Error: greet is not defined

C. greet(Alice)

D. Hello, Alice!

Solution

1. Step 1: Understand the declaration and implementation

   The declaration says greet takes a string and returns a string. The implementation returns a template string greeting.

2. Step 2: Evaluate the output of greet('Alice')

   Calling greet('Alice') returns the string Hello, Alice! without quotes in the output.

3. Final Answer:

   Hello, Alice! -> Option D

4. Quick Check:

   Function returns greeting string = Hello, Alice! [OK]

Hint: String return does not include quotes in output display [OK]

Common Mistakes:

• Including quotes around string output incorrectly
• Confusing function call with output text
• Assuming runtime error without implementation

4. Identify the error in this custom declaration file snippet:

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

medium

A. Missing semicolon after function declaration

B. Wrong module name syntax

C. Function parameters must be optional

D. Cannot export functions in declaration files

Solution

1. Step 1: Check syntax inside module declaration

   Each statement inside declaration files must end with a semicolon.

2. Step 2: Identify missing semicolon

   The function declaration line lacks a semicolon at the end.

3. Final Answer:

   Missing semicolon after function declaration -> Option A

4. Quick Check:

   Declaration statements need semicolons [OK]

Hint: Always end declarations with semicolons [OK]

Common Mistakes:

• Forgetting semicolons in declaration files
• Thinking exports are not allowed
• Misnaming modules with wrong quotes

5. You want to add a global type for a new property window.myAppVersion in a TypeScript project. Which custom declaration file snippet correctly adds this global property?

hard

A. declare module 'global' { interface Window { myAppVersion: string; } }

B. declare module 'window' { export const myAppVersion: string; }

C. declare global { interface Window { myAppVersion: string; } } export {};

D. interface Window { myAppVersion: string; }

Solution

1. Step 1: Understand how to add global types

   Use declare global block to augment global interfaces like Window.

2. Step 2: Check each option for correct syntax

   declare global { interface Window { myAppVersion: string; } } export {}; correctly declares global and extends Window interface, then exports empty to make it a module.

3. Final Answer:

   declare global { interface Window { myAppVersion: string; } } export {}; -> Option C

4. Quick Check:

   Global augmentation uses declare global + interface extension [OK]

Hint: Use 'declare global' to add global types safely [OK]

Common Mistakes:

• Trying to declare global types inside a module
• Omitting export {} to make file a module
• Declaring interface without 'declare global'