New unknown top type

TypeScript 3.0 introduces a new top type unknown.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.

Example

// In an intersection everything absorbs unknown
type T00 = unknown & null;  // null
type T01 = unknown & undefined;  // undefined
type T02 = unknown & null & undefined;  // null & undefined (which becomes never)
type T03 = unknown & string;  // string
type T04 = unknown & string[];  // string[]
type T05 = unknown & unknown;  // unknown
type T06 = unknown & any;  // any
// In a union an unknown absorbs everything
type T10 = unknown | null;  // unknown
type T11 = unknown | undefined;  // unknown
type T12 = unknown | null | undefined;  // unknown
type T13 = unknown | string;  // unknown
type T14 = unknown | string[];  // unknown
type T15 = unknown | unknown;  // unknown
type T16 = unknown | any;  // any
// Type variable and unknown in union and intersection
type T20<T> = T & {};  // T & {}
type T21<T> = T | {};  // T | {}
type T22<T> = T & unknown;  // T
type T23<T> = T | unknown;  // unknown
// unknown in conditional types
type T30<T> = unknown extends T ? true : false;  // Deferred
type T31<T> = T extends unknown ? true : false;  // Deferred (so it distributes)
type T32<T> = never extends T ? true : false;  // true
type T33<T> = T extends never ? true : false;  // Deferred
// keyof unknown
type T40 = keyof any;  // string | number | symbol
type T41 = keyof unknown;  // never
// Only equality operators are allowed with unknown
function f10(x: unknown) {
    x == 5;
    x !== 10;
    x >= 0;  // Error
    x + 1;  // Error
    x * 2;  // Error
    -x;  // Error
    +x;  // Error
}
// No property accesses, element accesses, or function calls
function f11(x: unknown) {
    x.foo;  // Error
    x[5];  // Error
    x();  // Error
    new x();  // Error
}
// typeof, instanceof, and user defined type predicates
declare function isFunction(x: unknown): x is Function;
function f20(x: unknown) {
    if (typeof x === "string" || typeof x === "number") {
        x;  // string | number
    }
    if (x instanceof Error) {
        x;  // Error
    }
    if (isFunction(x)) {
        x;  // Function
    }
}
// Homomorphic mapped type over unknown
type T50<T> = { [P in keyof T]: number };
type T51 = T50<any>;  // { [x: string]: number }
type T52 = T50<unknown>;  // {}
// Anything is assignable to unknown
function f21<T>(pAny: any, pNever: never, pT: T) {
    let x: unknown;
    x = 123;
    x = "hello";
    x = [1, 2, 3];
    x = new Error();
    x = x;
    x = pAny;
    x = pNever;
    x = pT;
}
// unknown assignable only to itself and any
function f22(x: unknown) {
    let v1: any = x;
    let v2: unknown = x;
    let v3: object = x;  // Error
    let v4: string = x;  // Error
    let v5: string[] = x;  // Error
    let v6: {} = x;  // Error
    let v7: {} | null | undefined = x;  // Error
}
// Type parameter 'T extends unknown' not related to object
function f23<T extends unknown>(x: T) {
    let y: object = x;  // Error
}
// Anything but primitive assignable to { [x: string]: unknown }
function f24(x: { [x: string]: unknown }) {
    x = {};
    x = { a: 5 };
    x = [1, 2, 3];
    x = 123;  // Error
}
// Locals of type unknown always considered initialized
function f25() {
    let x: unknown;
    let y = x;
}
// Spread of unknown causes result to be unknown
function f26(x: {}, y: unknown, z: any) {
    let o1 = { a: 42, ...x };  // { a: number }
    let o2 = { a: 42, ...x, ...y };  // unknown
    let o3 = { a: 42, ...x, ...y, ...z };  // any
}
// Functions with unknown return type don't need return expressions
function f27(): unknown {
}
// Rest type cannot be created from unknown
function f28(x: unknown) {
    let { ...a } = x;  // Error
}
// Class properties of type unknown don't need definite assignment
class C1 {
    a: string;  // Error
    b: unknown;
    c: any;
}