Declaration file syntax (.d.ts) in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a .d.ts file in TypeScript?

A .d.ts file provides type declarations for JavaScript code, allowing TypeScript to understand the types without the actual implementation code.

beginner

How do you declare a variable in a .d.ts file?

Use the <code>declare</code> keyword followed by the variable name and its type, for example: <code>declare const myVar: string;</code>

beginner

What keyword is used to declare a function signature in a .d.ts file?

The <code>declare</code> keyword is used before the function signature, for example: <code>declare function greet(name: string): void;</code>

intermediate

How do you declare a module in a .d.ts file?

Use the declare module 'module-name' { ... } syntax to describe the module's types and exports.

beginner

Can you include implementation code inside a .d.ts file?

No, .d.ts files only contain type declarations without any actual code implementation.

What does the declare keyword do in a .d.ts file?

A Imports external libraries

B Implements functions and variables

C Defines the shape of variables, functions, or modules without implementation

D Runs TypeScript code

The declare keyword tells TypeScript about the existence and type of variables, functions, or modules without providing their implementation.

Which file extension is used for TypeScript declaration files?

A .d.ts

B .tsx

C .ts

D .js

Declaration files use the .d.ts extension to provide type information without implementation.

Can you write executable code inside a .d.ts file?

A Yes, it runs like normal TypeScript

B No, it only contains type declarations

C Only for functions

D Only for variables

.d.ts files only describe types and do not contain executable code.

How do you declare a class in a .d.ts file?

A Use <code>function ClassName() { ... }</code>

B Use <code>class ClassName { ... }</code> without declare

C Use <code>interface ClassName { ... }</code>

D Use <code>declare class ClassName { ... }</code>

Classes in declaration files are declared with the declare keyword to describe their shape without implementation.

Which of the following can you NOT declare in a .d.ts file?

A Function implementations

B Functions

C Modules

D Variables

Declaration files only declare types and signatures, not implementations.

Explain the role of the .d.ts file in a TypeScript project and how it helps with JavaScript libraries.

Think about how TypeScript knows types for code it doesn't compile.

Describe how to declare a module and a function inside a .d.ts file.

Focus on the keywords and structure used for declarations.

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