[jez]

They're basically Hash literal types in Ruby. It's what TypeScript calls object types:

https://www.typescriptlang.org/docs/handbook/2/objects.html

(Ruby and JavaScript mean slightly different things by the word "object" so we chose a different word.)

Flow also has a distinction between exact and inexact object types:

https://flow.org/en/docs/types/objects/

where the difference is whether other, unspecified fields are allowed to hide in the object, or whether values of type `{foo: number}` must have only the `foo` field, and no other fields.