Why doesn't my TS Boolean override work for `any` or `unknown`?-CodePudding

I want Boolean(truthy) to yield a type of true instead of Boolean, and likewise for falsy => false

So I wrote this:

interface BooleanConstructor {
  <T extends false | 0 | '' | null | undefined>(value?: T): false;
  <T extends Record<any, any> | string | number | true>(value?: T): true;
  <T>(value?: T): boolean;
}

Works great, except for any and unknown. Any ideas?

const A = Boolean(42 as any);     // false ??
const B = Boolean(42 as unknown); // boolean
const A2 = Boolean('');           // false
const B2 = Boolean(0);            // false
const C = Boolean(42);            // true
const D = Boolean('hello');       // true
const E = Boolean(true);          // true
const F = Boolean(false);         // false
const G = Boolean(null);          // false
const H = Boolean(undefined);     // false
const I = Boolean([]);            // true
const J = Boolean({});            // true

ts playground with a local function instead, but same same.

CodePudding user response：

Boolean(42 as any) can match either overload (because any matches anything), so it will resolve whichever comes first. If you swap your overload order, you'll get true instead of false here.

For a hack that might work, see this playground. Basically, you need a new overload where T looks for a type that won't match any of your other overloads to go first, something like:

interface BooleanContstructor {
  <T extends { [`@hopefully-this-long-string-will-never-be-used-as-a-property`]: {} }>(value: T): boolean
  // ...

Or, do this.