Inconsistency with Rest Parameter in Function Type

🔎 Search Terms

rest parameters, parameter list, function parameters

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about parameters and rest parameters

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.4.5&ssl=5&ssc=12&pln=1&pc=1#code/C4TwDgpgBAysBOBLAdgcwGIFdkGNiIHtkoBeKACgDpqBDeVAZwC4oGEVUBtAXQEpSAfFABuBRABMA3AChpOImygQAHjQC2YADYQAjCzhI0WXPiKkKdVHtbs0-EkPnIGBbZU0FU5SzvcQ0wAAWvDLSKupauuQhQA

💻 Code

type StringFunction = (...args: string[]) => void;

const example1: StringFunction = (arg1: string) => console.log(arg1.length);

example1();

🙁 Actual behavior

This code does not display any type errors on any of the versions I tried, but will break at run time on the example1() call.

🙂 Expected behavior

Typescript should prevent the assignment of (arg1: string) => void to a variable of type (...args: string[]) => void since it is possible to have args be empty.

Additional information about the issue

I attempted to search for other issues related to this and came up empty handed. This is different from the "I'm allowed to use a shorter parameter list where a longer one is expected" issue which has drowned out a lot of my searching.

Thanks in advance for any help with this.

I found this in the existing tests:

TypeScript/tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignaturesWithRestParameters.ts

Line 16 in 79a8514

    
           a = (x: number) => 1; // ok, rest param corresponds to infinite number of params

This is a pretty old test though and it assumes strictNullChecks: false. I think it doesn't quite make sense in the strict world for this to be allowed. Rest params are effectively optional so why do we observe a difference here?

// error
const fn1: (a?: string) => void = (a: string) => {}

// ok?
const fn2: (...args: number[]) => void = (x: number) => {};

We can find some recent tests with strict: true that would break if the above wouldn't work:

TypeScript/tests/cases/compiler/assignmentToAnyArrayRestParameters.ts

Lines 6 to 14 in 79a8514

    
           function foo<T extends string[]>( 
        
               fa: (s: string, ...args: string[]) => string, 
        
               fb: (s: string, ...args: T) => string 
        
           ) { 
        
               const f1: (...args: any) => string = fa; 
        
               const f2: (...args: any[]) => string = fa; 
        
               const f3: (...args: any) => string = fb; 
        
               const f4: (...args: any[]) => string = fb; 
        
           }

At the very least f2 and f4 should error here - assuming that the issue at hand would get fixed.

rest param corresponds to infinite number of params

Yeah, no. It can be up to an infinite number of params, but a minimum of zero. i.e. the same reason fn1 is an error in your example.

I guess the reasoning here is the same as how an object with a string index signature is treated in some contexts as having all possible properties at once, which is only rectified by enabling noUncheckedIndexedAccess.

This is an intentional affordance for functions like Regex#replace or (throwback) AMD's define call.

In practice any time you alias a function into a position with rest args, the number of arguments you'll get is almost always determinable at the call site (e.g. by the number of groups in the regex, the number of module names passed to define, and so on)

I could swear we discussed this on GitHub (not CodePlex) but I can't find it - very hard to search for

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

I could swear we discussed this on GitHub

Maybe you were thinking of the discussion in #9352?

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Inconsistency with Rest Parameter in Function Type #58552

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	function foo<T extends string[]>(
	fa: (s: string, ...args: string[]) => string,
	fb: (s: string, ...args: T) => string
	) {
	const f1: (...args: any) => string = fa;
	const f2: (...args: any[]) => string = fa;
	const f3: (...args: any) => string = fb;
	const f4: (...args: any[]) => string = fb;
	}

Inconsistency with Rest Parameter in Function Type #58552

Description

🔎 Search Terms

🕗 Version & Regression Information

⏯ Playground Link

💻 Code

🙁 Actual behavior

🙂 Expected behavior

Additional information about the issue

Activity

Andarist commented on May 17, 2024

fatcerberus commented on May 17, 2024

RyanCavanaugh commented on Jun 14, 2024

RyanCavanaugh commented on Jun 14, 2024

typescript-bot commented on Jun 17, 2024

mkantor commented on Jul 4, 2025

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