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hasOwnProperty return type #41915

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hasOwnProperty return type#41915
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Awaiting More FeedbackThis means we'd like to hear from more people who would be helped by this featureSuggestionAn idea for TypeScript

Description

@WORMSS
Colin Richardson (WORMSS)
opened on Dec 10, 2020

Search Terms

hasOwnProperty

Suggestion

interface Object {
    hasOwnProperty<Key extends PropertyKey>(v: Key): this is this & Record<Key, unknown>;
}

Use Cases

To be able to use the property of an object after it has been tested for its existence.
Current behavior:

// Property 'testProp' does not exist on type 'object'.(2339)
obj.hasOwnProperty('testProp') && typeof arg.testProp === 'string'

Examples

https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgPICMBWEFmQbwFgAoZM5AeguQGEB7AWwYnGToFc8wALYAZ2QAbUCjB1kfCCnTtggsAFpQydnzgBzCADoS5ZNzh9UAdxAAFKHQAO0MAE8APAGkId5BAAekEABMBF61s7FzsAPgAKADcALmQQgEpYnn5kFOSBADJkACUcOigfZ1cAGhUQAGsQOlNQgG4SAF8SEhh2EFxgOhBkSD4wcLgodVi2yuqQeIJdcmAYZHDm0j1yexs6OcH1ZABeXeQAcjosHDB96eXkDKzN5ABCPZB2QUFz5avkTa0DI1MAmyh7OF9r0wH99vFXnp3qsIOsPkMtCC-js9vs+lBQOp9pRqAA5fIMOCCdxQSxQWJ-IIHJGWKzYnx0CACKp4Tz8PBdHp2GwHI7YXD7LThABMAGZRQBOCHEV6TIhLC7GODgPgAZTAGJA6gGCJp1ni9QVyCaxBNLTaHU5IOFOuGZTGpjlr1m80WFzIMLhN12215xwFkPI7xu919j2egbI72BTNBtOxyk2kcuWU9G11seRPoO6Mx2KoyFVcGYJLJFNpVJjfTByAZTOQLPcHnZbG6ML9-NOQrFkulsqmRr0SpV6s12s+eqsBteJrNxFa7TAnW6w7Aao1mPC6Niua1cuQUAgYHYUG66NqxtqQA

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. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
  • This feature would agree with the rest of TypeScript's Design Goals.

Activity

  1. WORMSS commented on Dec 10, 2020

    @WORMSS
    Colin Richardson (WORMSS)
    on Dec 10, 2020
    Author

    PS: if this can be 'fixed' for 'testProp' in obj that would be amazing too.

  2. MartinJohns commented on Dec 10, 2020

    @MartinJohns
    Martin Johns (MartinJohns)
    on Dec 10, 2020
    Contributor

    Duplicate of #18282.

  3. MartinJohns commented on Dec 10, 2020

    @MartinJohns
    Martin Johns (MartinJohns)
    on Dec 10, 2020
    Contributor

    if this can be 'fixed' for 'testProp' in obj that would be amazing too.

    This is tracked by #21732.

  4. WORMSS commented on Dec 10, 2020

    @WORMSS
    Colin Richardson (WORMSS)
    on Dec 10, 2020
    Author

    Duplicate of #18282.

    You say it's a duplicate, but it seems their example was a far inferior attempt?
    None of the examples in #18282 would be as seamless as this is this & Record<Key, unknown>
    Maybe due to 3 years ago when TS was not as mature.

  5. Ryan Cavanaugh (RyanCavanaugh)
    added
    Awaiting More FeedbackThis means we'd like to hear from more people who would be helped by this feature
    SuggestionAn idea for TypeScript
    on Dec 10, 2020

  6. Nokel81 commented on Jan 12, 2021

    @Nokel81
    Sebastian Malton (Nokel81)
    on Jan 12, 2021

    This would be amazing! Working with unknown is really annoying currently because even after doing an explicit check (what will happen at runtime) it still treats it as non existing.

    Take the following example:

    interface Arg {
    field1: string;
    field2: number;
}

function validateArg(val: unknown): val is Arg {
    if (val == null || typeof val !== "object") {
        return false;
    }

    val; // here `val` is `object`

    if (typeof val.field1 !== "string") {
        // ^-- error "field1" does not exist on "object"
    }

    if (!obj.hasOwnProperty("field1") || typeof val.field1 !== "string") {
        // ^-- error "field1" does not exist on "object"
        // ^^- hasOwnProperty doesn't change the deduced type of `val` at all
    }

    ...
}

    Currently the only solution is to define this function itself and use it or cast obj to Record<string, unknown> and go from there.

    This solution doesn't work for non-literals (as in it passes compilation but doesn't further narrow the type)./

  7. Malix-Labs commented on Nov 30, 2023

    @Malix-Labs
    Malix - Alix Brunet (Malix-Labs)
    on Nov 30, 2023

    Ryan Cavanaugh (@RyanCavanaugh) #41915 (comment)
    What feedback still needs to be awaited?

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