Boolean Conversion

Prefer to be explicit with boolean conversions. Use either the double bang notation (!!) or the Boolean() method. Implicit conversions, for example in if statements, are alright.

type User = {
    id: string
}

const user: User | null | undefined = {
    id: "01979ad0-8041-713d-b2e1-47219d90d881"
};

// ❌ WRONG - logically incorrect comparison (types differ)
if (user == true) { ... }
if (user === true) { ... }

// ❌ WRONG - creates the `Boolean` object -> objects are always truthy!
if (new Boolean(user)) { ... }

// ❌ WRONG - `isAuthenticated` is not a boolean
const isAuthenticated = user
// the following would be a typescript error
const isAuthenticated: boolean = user

// ✅ ALRIGHT - implicit conversion to a `boolean`
if (user) { ... }

// ✅ GOOD - explicit conversion
if (!!user) { ... }

// ✅ GOOD - explicit conversion
if (Boolean(user)) { ... }

// ✅ GOOD - explicit conversion
const isAuthenticated: boolean = Boolean(user)
const isAuthenticated: boolean = !!user

Type narrowing caveat

When using Boolean() method, keep in mind that type narrowing does not work as well:

type User = {
  id: string;
};

const narrowTypeBang = (user: User | null) => {
  const isUserDefined = !!user;

  if (isUserDefined) {
    return user.id; // `user` is of type `User` here, correctly narrowed type
  }
};

const narrowTypeBoolean = (user: User | null) => {
  const isUserDefined = Boolean(user);

  if (isUserDefined) {
    return user.id; // `user` is of type `User | null` here, not narrowed down, type error
  }
};