Lesson overview · Free interview practice

Type Guards

Implement custom type guards and narrowing.

Topics in the full lesson
  • Introduction to Type Guards and Type Narrowing
  • Understanding Type Guards
  • User-Defined Type Guards
  • Type Narrowing
  • Discriminated Unions
  • Exhaustiveness Checking
  • Advanced Type Guards
  • Best Practices
  • Exercises

Practice Problems

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TheoryEasy

Narrowing with the `in` Operator

Question

How does the in operator narrow types in TypeScript? When does it win over discriminated unions?

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Explain your reasoning or try an implementation before comparing answers.

TheoryMedium

Type Guards vs Type Assertions

Question

What's the difference between a type guard and a type assertion? When is each one safe to use?

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Explain your reasoning or try an implementation before comparing answers.

TheoryMedium

Assertion Functions: `asserts x is T`

Question

What is an assertion function in TypeScript? When does the asserts predicate fit better than a regular is type guard?

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Explain your reasoning or try an implementation before comparing answers.

TheoryMedium

Discriminated Unions: The Tagged Variant Pattern

Question

What is a discriminated union, and how does TS narrow it? Show the canonical exhaustive-switch pattern.

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Explain your reasoning or try an implementation before comparing answers.

TheoryMedium

User-Defined Type Guards with the `is` Predicate

Question

How do you write a custom type guard with the is predicate? Why does it narrow the type, and what's the lying-guard footgun?

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Explain your reasoning or try an implementation before comparing answers.

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