The unknown type

ℹ️ The 'unknown' type in TypeScript is the type-safe counterpart of 'any'. Unknown is assignable only to itself and any type. Unknown doesn't allow any operations without first asserting or narrowing.

unknown is the type-safe counterpart of any.

Anything is assignable to unknown, but unknown isn’t assignable to anything but itself and any without a type assertion or a control flow based narrowing. Likewise, no operations are permitted on an unknown without first asserting or narrowing to a more specific type.

The easiest way to understand unknown is to compare it with any. As we know any type is used to opt-out of type checking. By using any type we can perform any action:

let value: any;

value = true;          // ✅ OK
value = 42;            // ✅ OK
value = null;          // ✅ OK
value = undefined;     // ✅ OK
// And so on... We can assign anything

/** ⬇️ The same remains for using our value. We can do whatever we want. */
value.foo.bar;  // ✅ OK
value();        // ✅ OK
value[0][1];    // ✅ OK
new value();    // ✅ OK

Just like all types are assignable to any, all types are assignable to unknown:

let value: unknown;

value = true;          // ✅ OK
value = 42;            // ✅ OK
value = null;          // ✅ OK
value = undefined;     // ✅ OK

However, we can't simply perform any operations with our variable like it was with any type:

let value: unknown;

value.foo.bar;  // ❌ ERROR
value();        // ❌ ERROR
value[0][1];    // ❌ ERROR
new value();    // ❌ ERROR

// ================

if (typeof value === 'function') {
    value();    // ✅ OK
}

if (Array.isArray(value)) {
  value[0];    // ✅ OK
} 

Additionally, we can't assign unknown type to any other types except itself and any:

let unknownValue: unknown;
let anyValue: any;

let value1: unknown = unknownValue;   // ✅ OK
let value2: any = unknownValue;       // ✅ OK
let value3: unknown = anyValue;       // ✅ OK

let value4: boolean = unknownValue;   // ❌ ERROR
let value5: string = unknownValue;    // ❌ ERROR
let value6: object = unknownValue;    // ❌ ERROR
let value7: any[] = unknownValue;     // ❌ ERROR

Read more:

• typescriptlang.org | New Unknown Type