Open

Description
Suggestion
Ability to discriminate between union members when not all union members are known.
Use Cases
- redux actions: See Allow switch type guards #2214 (comment)
- This would enable us to discriminate on Node kinds without converting Node to a huge union (which slows down compilation too much). (Also, Node is effectively open-ended because we add new kinds fairly often.)
Examples
interface Shape {
unique kind: string;
}
interface Square extends Shape {
kind: "square";
size: number;
}
interface Circle extends Shape {
kind: "circle";
radius: number;
}
// other shapes may exist
function area(s: Shape) {
switch (s.kind) {
case "square":
return s.size * s.size;
case "circle":
return Math.PI * s.radius ** 2;
default:
return 0; // Or hand off to some other function that handles other Shape kinds
}
}
Checklist
My suggestion meets these guidelines:
- This wouldn't be a breaking change in existing TypeScript / JavaScript code
- This wouldn't change the runtime behavior of existing JavaScript code
- This could be implemented without emitting different JS based on the types of the expressions
- This isn't a runtime feature (e.g. new expression-level syntax)
Workarounds
Cast the general type to a union of known type.
function area(sIn: Shape) {
const s = sIn as Square | Circle;
...