Function returning Partial<T> allows incorrect objects

[nicolaslaplume]

Bug Report

🔎 Search Terms

Partial

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ

⏯ Playground Link

Playground link with relevant code

💻 Code

type Person = {
    name: string;
    age: number;
}
const person:Partial<Person> = {name: 'Dave', ags: 3};
const getPerson: ()=>Partial<Person> = ()=>({name: 'Dave', ags: 3});

🙁 Actual behavior

There is a single error, when assigning directly to person.

🙂 Expected behavior

There are two errors, with the same error about Object literal may only specify known properties, and 'ags' does not exist in type 'Partial<Person>' for the returned value of getPerson

[nicolaslaplume]

and it looks that doing a direct cast also prevents the error from showing:

const person:Partial<Person> = {name: 'Dave', ags: 3} as Partial<Person>;

[MartinJohns]

This is not a bug, and the objects are not incorrect. Additional properties are allowed.

The first error is due to excess property checks, which is more of a linter feature. You get the same behaviour in your second assignment, when you type your fat-arrow function accordingly (add a type annotation that it returns Partial<Person>).

Currently your function ()=>({name: 'Dave', ags: 3}) has no type annotation for the return type, so the return type is inferred. Then you assign this to a variable typed ()=>Partial<Person>, and the compiler checks if the type is assignable, which it is.

and it looks that doing a direct cast also prevents the error from showing:

A type assertion (often called a cast) is a way to tell the compiler "please ignore this obvious issue, I know what I'm doing".

[fatcerberus]

See also #12936 (proposal for Exact Types).

[jcalz]

Also duplicate of #37017 and quite a few others, and ultimately #241

[typescript-bot]

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