Mastering unknown vs never Types in TypeScript: Differences and Use Cases

unknown Type

• Represents any value, but is safer than any—you cannot use its value directly without type-checking or type assertion
• Use when you want to accept any type but enforce type-safety downstream

Example:

function logValue(value: unknown) {
  if (typeof value === "string") {
    console.log(value.toUpperCase());
  }
}

Key Points:

• Forces you to perform a type check before using the value
• Useful in libraries for handling user input, JSON parsing, etc.

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never Type

• Represents values that never occur
• Used as the return type for functions that never return (e.g., throw exceptions or have infinite loops)

Example:

function fail(message: string): never {
  throw new Error(message);
}

Key Points:

• Helpful for exhaustive checks (e.g., switch statements)
• Informs TypeScript that code after a never function is unreachable

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When to Use Which?

• Use unknown for any value you want type-checked later
• Use never for impossible states or functions never meant to return