TypeScript never and unknown Types

The never and unknown types are essential for type-safe programming in TypeScript. Here's how to use them.

Type Inference with never and unknown#

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Best Practices#

Use unknown When:
✓ Receiving external data (API, user input)
✓ Catch blocks
✓ JSON parsing
✓ Generic value containers

Use never When:
✓ Functions that throw or loop forever
✓ Exhaustive type checking
✓ Filtering union types
✓ Impossible code paths

Narrowing unknown:
✓ typeof for primitives
✓ instanceof for classes
✓ Type predicates for objects
✓ Assertion functions

Avoid:
✗ Using any instead of unknown
✗ Type assertions without validation
✗ Ignoring never in switch defaults
✗ Over-complicating type guards

unknown is the type-safe alternative to any - use it for values whose type is truly unknown and narrow with type guards. never represents impossible values - use it for exhaustive checks, functions that never return, and filtering types. Together, they enable robust type-safe programming while maintaining flexibility for dynamic values.

TypeScript never and unknown Types

The unknown Type#

unknown vs any#

Type Narrowing with unknown#

The never Type#

Exhaustive Checks#

never in Conditional Types#

unknown in API Responses#

Error Handling with unknown#

never as Return Type#

Type Inference with never and unknown#

never in Mapped Types#

Best Practices#

Conclusion#

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