Model API States with Unions
Model idle, loading, success, and error states with a discriminated union. Prevent impossible combinations and exhaustively render each state.
Examples
Compare approaches
Baseline approach
Begin by explaining the core mechanism, stating assumptions, and walking through one concrete example.
Time: Depends on design · Space: Depends on design
type State<T> =
| { status: "idle" }
| { status: "loading" }
| { status: "success"; data: T }
| { status: "error"; message: string };Strong answer
A discriminant makes available fields depend on the state. An exhaustive switch with a never check catches newly added variants.
Time: Discuss operation costs · Space: Discuss retained state
type State<T> =
| { status: "idle" }
| { status: "loading" }
| { status: "success"; data: T }
| { status: "error"; message: string };Common traps
- Optional data and error fields permit inconsistent states.
- State assumptions and justify trade-offs rather than memorizing a single answer.