Flow-Predicate-Checks.js

type HasFoo = { kind: "Foo" }
type HasBar = { kind: "Bar", baz: string }

class Other {
  baz = "cool"
}

function hasBar(obj: HasFoo | HasBar | Other): boolean %checks {
  return obj instanceof Other
}

declare var z: HasFoo | HasBar | Other

if (hasBar(z)) {
  z.baz
}

type MaybePromise<T> = Promise<T> | T

declare function isPromise(obj: mixed): boolean %checks(
  obj instanceof Promise
)
function isPromise(obj) {
  return obj && typeof obj.then === 'function'
}

declare var x: mixed;

if (isPromise(x)) {
  x.then(x => x)
}


const y = { then: x => x }

if (isPromise(y)) {
  y.then(x => x)
}