hasOwnProperty return type

[WORMSS]

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.

[WORMSS]

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

[MartinJohns]

Duplicate of #18282.

[MartinJohns]

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

This is tracked by #21732.

[WORMSS]

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.

[Nokel81]

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)./

[Malix-Labs]

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