Lesson overview · Free interview practice

Declarations

Work with declaration files and ambient modules.

Topics in the full lesson
  • Introduction to Declaration Files
  • Creating Declaration Files
  • Ambient Declarations
  • Consuming JavaScript Libraries
  • Writing Declaration Files for JavaScript Libraries
  • Advanced Type Declarations
  • Best Practices for Declaration Files
  • Common Issues and Troubleshooting
  • Exercises

Practice Problems

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TheoryEasy

The `declare` Keyword: Marking Type-Only Declarations

Question

What does the declare keyword do? Where does it appear, and what are the rules around using it?

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Explain your reasoning or try an implementation before comparing answers.

TheoryMedium

`.d.ts` Declaration Files: Purpose and Generation

Question

What is a .d.ts declaration file in TypeScript? When are they generated automatically, and when do you write them by hand?

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TheoryMedium

Global vs Module Declarations

Question

What's the difference between a global declaration and a module declaration in TypeScript? Show when each one fits.

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TheoryMedium

Writing a `.d.ts` File for an Untyped Module

Question

Walk through writing a .d.ts declaration file for an untyped JavaScript library. What's the canonical structure, and how do you handle different export patterns?

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Explain your reasoning or try an implementation before comparing answers.

TheoryMedium

Ambient Declarations: `declare` for External Code

Question

What are ambient declarations in TypeScript? When does declare appear, and what does it tell the compiler?

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Explain your reasoning or try an implementation before comparing answers.

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