microsoft / microsoft/TypeScript

Union with non-distinct discriminating property breaks inference somewhat (confusing error messages, invalid intellisense)

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#40,934 8 comments 1 reaction 1 assignee View on GitHub

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Domain: Error Messages Experience Enhancement
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Description

TypeScript Version: 4.0.2

Search Terms: discriminated union, error messages

Code

type Action = {
    type: 'action1';
    payload: {
        property1: string;
    };
} | {
    type: 'action1';
    payload: {
        property2: number;
    };
} | {
    type: 'action2';
    payload: {
        property3: boolean;
    };
}

const action: Action = { // Actual error is here
    type: 'action1',
    payload: {
        property3: true // Expected error here
    }
}

Expected behavior:

Expected error at action.payload.property3 declaration, perhaps something like:

Object literal may only specify known properties, and 'property3' does not exist in type '{ property1: string; } | { property2: number; }'.

Also, once the object has been refined by the type action1, intellisense should only provide property1 and property2 as a suggestion when declaring the payload.

Actual behavior:

An error message at the first line of the declaration:

Type '{ type: "action1"; payload: { property3: true; }; }' is not assignable to type 'Action'.
  Type '{ type: "action1"; payload: { property3: true; }; }' is not assignable to type '{ type: "action2"; payload: { property3: boolean; }; }'.o
      Type '"action1"' is not assignable to type '"action2"'.(2322)

Also, once the object has been refined by the type action1, intellisense still provides property3 as a suggestion when declaring the payload.

Playground Link: https://www.typescriptlang.org/play?#code/C4TwDgpgBAggxsAlgewHZQLxQN4CgoFSiQBcUA5AIYIqoCM5A3PoWJSADbKUAmZehQVDAAnZJBGg6ZAM7ARiVAHNmggL7M1UAD44WBYhDJUaaBqtbsuvfvqGjxESSABMZVAFcAtgCMnFgg1cLV0BQkNjaiQ0FyY7Nk5uPj0hVjEJUABmMh9kZA4ISlQAqCC1XFw4NDkoKNoyeGj0LDCDcCMKOrNyABp4qyTbVIIHDJBsohEPCDtytSA

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