为什么此 TypeScript 详尽的开关检查不起作用？

Question

Why does the following switch default case not lead to an exhaustive check where item is correctly identified as never?

enum Type {
    First,
    Second
}

interface ObjectWithType {
    type: Type;
}

const array: ObjectWithType[] = [];

for (const item of array) {
    switch (item.type) {
        case Type.First:
            break;
        case Type.Second:
            break;
        default: {
            const unhandledItem: never = item;
            throw new Error(`Unhandled type for item: ${unhandledItem}`);
        }
    }
}

Typescript playground link

Answer 1

It's not working because it's not the item that has never type, it's the item.type .

If you modify your code like so, it will work:

const unhandledItem: never = item.type;
// −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−^^^^^

Answer 2

As @thedude said, the problem is that you're using item instead of item.type .

You can correct the problem and make the code simpler by grabbing type from item early and then using that; in the default , the type of type will automatically be narrowed to never :

for (const item of array) {
    const {type} = item;
    switch (type) {
        case Type.First:
            break;
        case Type.Second:
            break;
        default: {
            throw new Error(`Unhandled type for item: ${type}`);
            // The type here is automatically narrowed  ^^^^  to `never`
        }
    }
}

Playground link