JuniorTheoryCommon

Which type does `s` have on the line `return "paid: " + s.total;`?

TypeScript

type OrderState =
    | { kind: "new" }
    | { kind: "paid"; total: number }
    | { kind: "shipped"; track: string };

function statusText(s: OrderState): string {
    switch (s.kind) {
        case "paid":
            return "paid: " + s.total;
    }
    return "other";
}

{ kind: "paid"; total: number; }

Expecting s to keep its declared type OrderState: the case compares the discriminant, and inside it s is the paid variant with total.
Confusing s with s.kind: the check narrows the whole object, so s is { kind: "paid"; total: number; }, not "paid".

Which type does s have after the switch, on the line return "other";?
How would you make tsc report a state that the switch forgot?

Back to questions Open topic in roadmap →Answer it yourself — Quiz

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JuniorTheoryCommon

Which type does `s` have on the line `return "paid: " + s.total;`?

TypeScript

type OrderState =
    | { kind: "new" }
    | { kind: "paid"; total: number }
    | { kind: "shipped"; track: string };

function statusText(s: OrderState): string {
    switch (s.kind) {
        case "paid":
            return "paid: " + s.total;
    }
    return "other";
}

{ kind: "paid"; total: number; }

Expecting s to keep its declared type OrderState: the case compares the discriminant, and inside it s is the paid variant with total.
Confusing s with s.kind: the check narrows the whole object, so s is { kind: "paid"; total: number; }, not "paid".

Which type does s have after the switch, on the line return "other";?
How would you make tsc report a state that the switch forgot?

Back to questions Open topic in roadmap →Answer it yourself — Quiz

Which type does s have on the line return "paid: " + s.total;?

Which type does s have on the line return "paid: " + s.total;?

Which type does `s` have on the line `return "paid: " + s.total;`?

Which type does `s` have on the line `return "paid: " + s.total;`?