TS: any, unknown, never, undefined - paulip114/blog GitHub Wiki

🧠 When to Use unknown, any, never, and undefined in TypeScript

Type	Description	When to Use
any	Bypasses type checking completely	Avoid when possible. Use when migrating JS to TS or working with 3rd-party code where types are unclear.
unknown	Similar to any but forces type-checking before use	Use when you're accepting input (e.g., from user, API, JSON) and want to validate it before using.
never	Represents values that should never occur	Use for functions that throw or switch cases that should be impossible (e.g., exhaustive enums).
undefined	A value that hasn’t been assigned	Use for optional parameters or values that may not exist (e.g., foo?: string).

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🔍 1. any – Disable TypeScript

let value: any = 'hello'
value.toFixed() // TS doesn't complain, but this will crash!

✅ Use only if you're dealing with dynamic data and you want to opt out of type safety.

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🔍 2. unknown – Safer than any

let input: unknown = getUserInput()
if (typeof input === 'string') {
  console.log(input.toUpperCase()) // ✅ Safe to use
}

✅ Use unknown when accepting external data (e.g., JSON.parse, user input) and want to force validation before use.

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🔍 3. never – Should never happen

function fail(): never {
  throw new Error('This always throws')
}

function exhaustiveCheck(x: never) {} // used in exhaustive switch

✅ Use never for:

• Functions that never return
• Exhaustive switch checks (e.g., all enum values handled)

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🔍 4. undefined – A value not set

let user: { name: string, age?: number }

✅ Use when:

• A property or argument might not be provided
• You want to allow intentional absence

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🧪 Summary Table

Type	Safe?	Typical Use Case
any	❌	Disable type checking
unknown	✅	External input validation
never	✅	Throwing functions, exhaustive switches
undefined	✅	Optional values, return type when no value