XOR in union allows both

[jakobrosenberg]

TypeScript Version: 2.0.0

Code

interface Project { projectId: string }
interface Organization { organizationId: string }

type ProjectOrOrganization = Project | Organization

function findTask(query: ProjectOrOrganization) {
    // Tasks.find(query)
}

findTask({ projectId: 'foo' })
// should work

findTask({ organizationId: 'bar' })
// should work

findTask({ projectId: 'foo', organizationId: 'bar' })
// should not work, but does

findTask()
// should not work

Expected behavior:
findTask({ projectId: 'foo', organizationId: 'bar' }) should not work
Expected behavior based on a post by Ryan Cavanaugh (@RyanCavanaugh)

Actual behavior:
findTask({ projectId: 'foo', organizationId: 'bar' }) works

Ryan Cavanaugh (@RyanCavanaugh) wrote a reply here which implies that or-unions work as XOR.

[ahejlsberg]

A type T is assignable to a union type U if T is assignable to at least one of the constituents of U. In your example, the type { projectId: string, organizationId: string } is assignable to both of the constituents and therefore assignable to the union type. Unions don't imply exclusivity among the types, but if you really want to ensure that a type doesn't have a specific member, you can define that member as an optional property of some marker type that never occurs in the wild:

interface Missing { __missing__: void }
interface Project { projectId: string, organizationId?: Missing }
interface Organization { organizationId: string, projectId?: Missing }

In 2.0 you could use type undefined instead of the marker type (before 2.0 it wasn't possible to explicitly reference the undefined type).