I have a TypeScript interface like this:

export interface User {
  firstName?: string;
  lastName?: string;
  displayName?: string;
  photoURL?: string;
  email?: string;
  phoneNumber?: string;
  uid: string;
}

And I have a variable that is typed like this:

const user: boolean | User

Now I want to narrow it down using this:

if (typeof user === 'User') {
  //Do some things with the User object that are not possible with a boolean
}

But I am getting this TypeScript error:

This condition will always return 'false' since the types '"string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"' and '"User"' have no overlap. ts(2367)

I have also tried this:

if (user instanceof 'User') {
   //Do some things with the User object that are not possible with a boolean
}

But then it throws this TypeScript error:

The right-hand side of an 'instanceof' expression must be of type 'any' or of a type assignable to the 'Function' interface type. ts(2359)

How do I narrow the variable down for the User type?

CodePudding user response：

The typeof operator evaluates to expressions which include an enumeration of strings, amongst which "User" will never be included.

Reverse your conditional logic:

TS Playground

if (typeof user !== 'boolean') {
  user; // User
}
else {
  user; // boolean
}

While this works for your case, where the discrimination is binary, in the case where more types are possible, you will find that using a type predicate is quite helpful.