Unions and Intersection types

Intersection types

1interface BaseConfig { version: string; name: string; }
2
3interface DynamicConfig { fromFile: string; }
4
5interface StaticConfig { configuration: object; }
6
7type Configuration = (StaticConfig | DynamicConfig) **& BaseConfig**;

1const config: Configuration = {  
2   version: '1.0',  
3   name: 'myDynamicConfig',  
4   fromFile: './config.json'  
5}; // this is a DynamicConfig
6
7const config: Configuration = {  
8   version: '1.0',  
9   name: 'myStaticConfig',  
10   configuration: { ... }  
11} // this is a StaticConfig

The Discriminated Unions

1. Types that have a common, singleton type property — the discriminant.
2. Then, a type alias that takes the union of those types — the union.
3. Finally, a type guard on the common property (on the discriminant).

• The discriminant is the type property, shared by both Car and Moto types.
• The Vehicule type is the union type.
• Finally, the switch type guard is based on the type property (on the discriminant).

• typing redux reducers (which are basically switches on a action type discriminant property).

Mapped types

• keyof User will be a type that would be any value in key of User, so the following union type: ‘first_name’ | ‘last_name’ | ‘city’ | ‘twitter_handle’

• K in will give us an iterator of User properties union type.
• Finally, User[K], called a index type, will bring us the type of the K index of User.
  In short, when K is city, then User[K] will be CityShortFormat.

1type Partial<T> = { \[P **in** **keyof** T\]?: T\[P\]; }  
2type LocalUser = Partial<User>;

Conditionals

• Exclude<T, U> – “Exclude from T those types that are assignable to U.”

Conclusion

• Advanced Types · TypeScript

• andnp/SimplyTyped