Correct the section about discriminated union types #183
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The previous explanation was incorrect. An object can not be at the same time
`{ value: "foo" }` and `{ value: [number, number] }`, the two are incompatible.
Instead, this is a design limitation of TypeScript compiler in that it does not narrow
the type of the parent object after narrowing a property.
microsoft/TypeScript#30506 (comment)
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| Even though we have narrowed based on `event.value`, the logic doesn't filter up and sideways to `event.target`. This is because a union type `UserTextEvent | UserMouseEvent` could be BOTH at once. So TypeScript needs a better hint. The solution is to use a literal type to tag each case of your union type: |
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I think the example should not be deleted. It's a good illustration. But you are correct, the language here does sound confusing.
You can move it below the actual example and collapse it.
You can change the explanation to "
Take care: TypeScript does not narrow the type of a Discriminated Union on the basis of typeof checks. The type guard has to be on the value of a key and not it's type.
microsoft/TypeScript#30506 (comment)
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An object can not be at the same time
{ value: "foo" } and { value: [number, number] }, the two are incompatible.
Correct, "foo" & [number, number] would be evaluated to make little sense. However, we use a union not an intersection so the type is never"foo" | [number, number].
The example works in the typescript playground. Could you demonstrate with example code (+config and version) on the typescript playground that this example isn't working?
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@eps1lon, your response is unrelated to my PR description, I never talked about an intersection. I have pointed out what I consider to be a wrong explanation in case you had missed it before. If you find it useful, feel free to improve your materials further. If you find them useless, I don’t mind, feel free to ignore. |
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Thank you @denis-sokolov for this PR. Your work is merged in #205 |
The previous explanation was incorrect. An object can not be at the same time
{ value: "foo" }and{ value: [number, number] }, the two are incompatible.Instead, this is a design limitation of TypeScript compiler in that it does not narrow
the type of the parent object after narrowing a property.
microsoft/TypeScript#30506 (comment)