Anonymous Function Type Not Constraining Object Return Value

[atpur-rafif]

🔎 Search Terms

anonymous function type, return object value

🕗 Version & Regression Information

Same behavior for version 3.9.7, 4.9.5, 5.5.4, 5.6.0-beta and v5.6.0-dev.20240810.

⏯ Playground Link

https://www.typescriptlang.org/play/?jsx=0&ts=5.5.4#code/C4TwDgpgBAShwFcBOA7AagQwDYOgXigG8okAuKFBAWwCMIkoBfAKFEigAUMkMrMd8RKGHKVa9Js2YAzBCgDGwAJYB7FFGkAKMN17kuPPtlwBKcnESp+uIswCQSeMnWF7D8jsMA6MABo3APQBUAACwADOALQQAB6QitFISCpIbvTJZFAAROkpWf52jADczCxarlCVwuQADAVBoRHRcRAJualVUO3kOUl5pSZSbNAAggBiUATaulT6M9YQJpMAfLBOVsYQJfJq4cBQGNLk45NQ04akBrwLS3irrg7rLm6Znrw+BXYNYVGx8cCJDJpPqZXoZfL2YqlEqHTQVKoiKB1ezfJp-VoA9rMTrdbLtLIDIA

💻 Code

type ReturnValue = { r: number }
type ParamValue = { p: number }

function f(param: ParamValue): ReturnValue {
	return {
		r: param.p,
		// @ts-expect-error
		error: "error",
	};
}
f({
    p: 0,
	// @ts-expect-error
    error: "error"
})

type AF = (param: ParamValue) => ReturnValue;
const af: AF = (param:ParamValue) => {
	return {
		r: param.p,
		// @ts-expect-error
		error: "error",
	};
};
af({
    p: 0,
	// @ts-expect-error
    error: "error"
})

🙁 Actual behavior

At third // @ts-expect-error (return object of function af) giving error because it's not used. Since there is no error at that line.

🙂 Expected behavior

Typescript should give error about extraneous error property, since for non-anonymous type function f gave that error. This same error happen for parameter of both af and f function. I assume this should be the case for return value too.

[MartinJohns]

This is working as intended. Your function lacks a return type annotation, so the return type is inferred. The inferred type is then checked for assignability, where excess property checks don't happen anymore.

There are many issues about this.

[jcalz]

Duplicate #241 and others

[atpur-rafif]

This is working as intended. Your function lacks a return type annotation, so the return type is inferred. The inferred type is then checked for assignability, where excess property checks don't happen anymore.

There are many issues about this.

I tried to remove annotation for parameter at anonymous function af. So, if it's the case (about lack of annotation), then at last // @ts-expect-error (when calling function af), it should not throwing any error, because the parameter is inferred. But it still throwing the error.

type AF = (param: ParamValue) => ReturnValue;
const af: AF = (param) => {
	return {
		r: param.p,
		// @ts-expect-error
		error: "error",
	};
};
af({
    p: 0,
	// @ts-expect-error
    error: "error"
})

[typescript-bot]

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