Lesson overview · Free interview practice

Conditional Types

Master mapped and conditional type expressions.

Topics in the full lesson
  • Introduction to Mapped and Conditional Types
  • The keyof Operator
  • The typeof Operator
  • Mapped Types
  • Conditional Types
  • Distributive Conditional Types
  • Built-in Utility Types
  • Advanced Examples
  • Best Practices
  • Exercises

Practice Problems

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TheoryMedium

`typeof` at the Type Level

Question

How does typeof work in a type context? Show how it differs from runtime typeof and the common use cases.

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TheoryMedium

Conditional Type Use Cases

Question

What are the canonical use cases for conditional types? Show four real patterns and when they're the right tool.

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TheoryMedium

Conditional Types: `T extends X ? Y : Z`

Question

What is a conditional type in TypeScript? Show the T extends X ? Y : Z syntax and explain when to reach for one.

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TheoryHard

Distributive Conditional Types

Question

What are distributive conditional types in TypeScript? Walk through the mechanism step by step, then show the bracket-wrap trick that suppresses distribution.

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Explain your reasoning or try an implementation before comparing answers.

TheoryHard

The `infer` Keyword

Question

What does the infer keyword do? Show how it extracts types from positions inside conditional types.

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