lib.es5.d.ts: Default ArrayConstructor generic parameter to undefined

Josh Ghoulberg 👻 (@JoshuaKGoldberg)

⚙ Compilation target

ES2015

⚙ Library

lib.es5.d.ts

Missing / Incorrect Definition

The two constructor declaration lines without generics (new (arrayLength?: number): any[]; and (arrayLength?: number): any[];)

TypeScript/src/lib/es5.d.ts

Lines 1488 to 1497 in 903e82b

    
           interface ArrayConstructor { 
        
               new (arrayLength?: number): any[]; 
        
               new <T>(arrayLength: number): T[]; 
        
               new <T>(...items: T[]): T[]; 
        
               (arrayLength?: number): any[]; 
        
               <T>(arrayLength: number): T[]; 
        
               <T>(...items: T[]): T[]; 
        
               isArray(arg: any): arg is any[]; 
        
               readonly prototype: any[]; 
        
           }

Suggestion:

interface ArrayConstructor {
-   new (arrayLength?: number): any[];
-   new <T>(arrayLength: number): T[];
+   new <T = undefined>(arrayLength: number): T[];
    new <T>(...items: T[]): T[];
-   (arrayLength?: number): any[];
-   <T>(arrayLength: number): T[];
+   <T = undefined>(arrayLength: number): T[];
    <T>(...items: T[]): T[];
    isArray(arg: any): arg is any[];
    readonly prototype: any[];
}

Sample Code

Using the @typescript-eslint/no-unsafe-assignment ESLint rule

const arr: undefined[] = Array(10); // 💥 Unsafe assignment of type `any[]` to a variable of type `undefined[]`. eslint (@typescript-eslint/no-unsafe-assignment)

Documentation Link

In some operations, empty slots behave as if they are filled with undefined.

const arr = [1, 2, , , 5]; // Create a sparse array

// Indexed access
console.log(arr[2]); // undefined

// For...of
for (const i of arr) {
  console.log(i);
}
// Logs: 1 2 undefined undefined 5

// Spreading
const another = [...arr]; // "another" is [ 1, 2, undefined, undefined, 5 ]

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Indexed_collections#sparse_arrays:~:text=In%20some%20operations%2C%20empty%20slots%20behave%20as%20if%20they%20are%20filled%20with%20undefined.

Since TypeScript does not have a primitive for "empty slot", undefined seems like the closest default.

cc Josh Ghoulberg 👻 (@JoshuaKGoldberg) Brad Zacher (@bradzacher)

Workaround

To address the problem in the @typescript-eslint/no-unsafe-assignment ESLint rule without changing the lib.es5.d.ts declarations, pass in a generic type argument undefined to the Array constructor (which I'm arguing should be the default):

const arr: undefined[] = Array<undefined>(10);

Somewhat related: #24834

Your suggestion would be quite a breaking change.

Since TypeScript does not have a primitive for "empty slot", undefined seems like the closest default.

I don't see how this is relevant at all to your suggested change.

I don't see why someone would initialize an array with a length except in the rare case where they were about to fill it. This would break a number of cases, especially in JS where you can't easily specify type arguments.

My original motivation was actually that - spreading a newly-initialized array into a new array literal:

typescript-eslint Playground

[
  ...Array(5) // 💥 Unsafe spread of an `any` value in an array. ESLint @typescript-eslint/no-unsafe-assignment 
].map((val, index) => index)

Providing the undefined type argument makes this problem disappear:

typescript-eslint Playground

[...Array<undefined>(5)].map((val, index) => index)

Upon first thinking about it, it seemed like a logical default to me.

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

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lib.es5.d.ts: Default ArrayConstructor generic parameter to `undefined` #59379

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	interface ArrayConstructor {
	new (arrayLength?: number): any[];
	new <T>(arrayLength: number): T[];
	new <T>(...items: T[]): T[];
	(arrayLength?: number): any[];
	<T>(arrayLength: number): T[];
	<T>(...items: T[]): T[];
	isArray(arg: any): arg is any[];
	readonly prototype: any[];
	}

lib.es5.d.ts: Default ArrayConstructor generic parameter to undefined #59379

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karlhorky commented on Jul 21, 2024

Workaround

MartinJohns commented on Jul 21, 2024

RyanCavanaugh commented on Jul 29, 2024

karlhorky commented on Jul 29, 2024

typescript-bot commented on Aug 1, 2024

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lib.es5.d.ts: Default ArrayConstructor generic parameter to `undefined` #59379