What does {| ... |} mean in javascript? [duplicate]

Question

Reading through a javascript codebase, I encounter a block of code that looks like

export type RouteReducerProps = {|
  error?: Error,
  isResolving: boolean,
  isResolved: boolean,
  hasFailed: boolean,
|};

This looks like a javascript object literal definition, but I've never seen a javascript object literal with vertical bars inside the curly braces. What do those do?

Answer 1

This syntax is specific to Flow, a static type checker for javascript. It defines an Exact Object Type, which is an object for which a only a few specified keys are valid. In the example above, only the keys 'error?', 'isResolving', 'isResolved', and 'hasFailed' can be defined on the object RouteReducerProps.