Possible to extend types in Typescript

Question

Say I have the following type   type Event        name  string     dateCreated  string     type  string      I now want to extend this type  i e   type UserEvent extends Event        UserId  string       This doesn t work  How can I do this

User · Answer

you can intersect types    type TypeA         nameA  string     type TypeB         nameB  string     export type TypeC   TypeA  amp  TypeB    somewhere in you code you can now do   const some  TypeC         nameB   B       nameA   A

User · Answer

The keyword extends can be used for interfaces and classes only   If you just want to declare a type that has additional properties  you can use intersection type   type UserEvent   Event  amp   UserId  string    UPDATE for TypeScript 2 2  it s now possible to have an interface that extends object-like type  if the type satisfies some restrictions   type Event        name  string     dateCreated  string     type  string     interface UserEvent extends Event      UserId  string       It does not work the other way round - UserEvent must be declared as interface  not a type if you want to use extends syntax   And it s still impossible to use extend with arbitrary types - for example  it does not work if Event is a type parameter without any constraints

User · Answer

May be below approach will be helpful for someone TS with reactjs  interface Event      name  string     dateCreated  string     type  string     interface UserEvent lt T gt  extends Event lt T gt        UserId  string

User · Answer

What you are trying to achieve is equivalent to  interface Event      name  string     dateCreated  string     type  string     interface UserEvent extends Event      UserId  string       The way you defined the types does not allow for specifying inheritance  however you can achieve something similar using intersection types  as artem pointed out

User · Answer

You can also do  export type UserEvent   Event  amp    UserId  string

[javascript] Possible to extend types in Typescript?

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