Spreading an object causes target type to have properties from prototype chain while in runtime it does not

[msand]

🔎 Search Terms

"spread interface prototype chain"
"spread native classes includes properties from prototype chain"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about spreading objects and properties from prototype chain

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYewdgzgLgBAhjAvDMBTA7jAsnADgHmgCcBLMAcwBoZizyA+ACgEoBuAKHdElgCMkYAbwB0ouAF8O7AGYBXMMCglwMYIwC2eAFzY8hKKQrVaFesyHsYq8BBAAbVMLshyGvMIDWqAJ4QWbdnFONV42IA

💻 Code

const a = new Map<string, string>();

const b = {...a};

function c(map: Map<string, string>) {
  console.log(map.keys());
}

c(b);

🙁 Actual behavior

Typescript compiles without errors. It computes that the type of b satisfies the Map interface. Calling the function c with b as the only argument causes map.keys() to crash at runtime.

[ERR]: "Executed JavaScript Failed:" 
[ERR]: map.keys is not a function 

🙂 Expected behavior

Spreading into an object should only include own enumerable properties from the spread values in the type, nothing from the prototype chain.

Additional information about the issue

Is there any way to get the latest version of TypeScript to complain about the function call?

[MartinJohns]

Duplicate of #9726.

[msand]

Thanks @MartinJohns
So it seems there is no workaround for this in the case of types defined in TypeScript/lib declaration files.

This was causing some pain recently when refactoring some code and changing some interfaces; ts didn't complain, and I thought the refactoring was done, but then I got crashes at runtime.

[MartinJohns]

There is this eslint rule:.https://typescript-eslint.io/rules/no-misused-spread/

And generally it's good practice to have at least one non-public member in your own classes.

[msand]

@MartinJohns, wonderful, thank you!

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.