What is the difference between unknown and any? | TypeScript Interview Questions

What You'll Learn

any type
unknown type
When to use each

any Type

Disables type checking entirely.

code.tsTypeScript

let value: any = "hello";

// No errors, but dangerous!
value.foo.bar.baz;
value();
value.toUpperCase();
value = 42;

unknown Type

Type-safe counterpart to any. Requires type checking before use.

code.tsTypeScript

let value: unknown = "hello";

// Errors! Must check type first
value.toUpperCase(); // Error
value.foo;           // Error

// After type checking, OK
if (typeof value === "string") {
  value.toUpperCase(); // OK
}

Comparison

Feature	any	unknown
Assignable to	Everything	Only unknown/any
Operations	All allowed	Must narrow first
Type safety	None	Preserved

When to Use

code.tsTypeScript

// Use unknown for values from external sources
async function fetchData(): Promise<unknown> {
  const response = await fetch("/api");
  return response.json();
}

// Use any only when migrating JS or with problematic libs
declare const legacyLib: any;