Type predicate tips

⬅️ Execute Program Advanced TypeScript

Type predicate tips

Keep these two points in mind to prevent bugs when using type predicates:

• 1. TypeScript trusts us to implement type predicates correctly. Test your predicate functions well to make sure that they do what you think they do.
  2. Type predicates can take unions as their arguments, or they can take unknown. In most cases, unknown is the safest and most flexible option.

Indexing types with union types

• If User is {name: string, age: number}, then User['name' | 'age'] is string | number.

type User = {
  name: string
  age: number
};
 
type NameOrAge = User['name' | 'age'];
 
const user = {
  name: 'Amir',
  age: 36,
};
 
type NameOrAge = (typeof user)['name' | 'age'];
 
const nameOrAge: NameOrAge = 'Betty';
nameOrAge;

Distributive property:

type User = {name: string, age: number};
type PropertyTypes1 = User['name' | 'age'];
type PropertyTypes2 = User['name'] | User['age'];