CODEWITHSUNDEEP
flowtype
kristian
kristian

Reputation: 53

Flow Disjoint Unions with a default catch all

I'm wondering if it's possible to create a disjoint union type with a catch all. My idea if that if say the objects 'type' field is a certain value then I know I will have more data available.

I'm thinking it will look something like this:

type MyObject =
  | { type: 'extra_details', moreDetails: {here: 'it is'} }
  | { type: string }

Thanks!

Upvotes: 0

Views: 156

Answers (1)

Red Mercury
Red Mercury

Reputation: 4310

There is a chapter about Tagged unions in the docs. https://flowtype.org/docs/dynamic-type-tests.html#tagged-unions

But a check like

  function foo(x: MyObject) {
      if (x.type === 'extra_details') {
         // ...extra details code-path
      }
  }

is not sufficient to prove to flow that in this code-path, x can only be the extraDetails type. The check satisfies both types of the union. {type: string} is a more general case of {type: 'extra_details'}. You can can however check for the existence of the extra fields instead.

  if (x.moreDetails) {

  }

Upvotes: 1

Related Questions