hasOwnProperty return type

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.

Start with the suggested Object.hasOwnProperty declaration and the linked TypeScript Playground example, then inspect how the compiler narrows a value after this call. Done means the example type-checks with the proposed predicate without changing emitted JavaScript, with regression coverage for the behavior.