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.