Type Guard Issue with Array.prototype.fill, Array Constructor

Typegurd Issue with Array.prototype.fill, Array Constructor

TypeScript Version: 3.5.1

Search Terms:

• Array.prototype.fill
• fill
• ArrayConstructor
• array implicit any

Code

const foo: number[] = new Array(3).fill("foo"); // accepted

Actual behavior: This code above is accepted because new Array(3) returns any[], and so fill("foo") returns any[].

I know giving type explicitly to Array like

const foo: number[] = new Array<number>(3).fill("foo"); // error

would work, but I believe the compiler should reject the first one. (This is TypeScript.)

Expected behavior:

Option A. Array.prototype.fill returns narrow type replace declaration of Array.fill in lib.es2015.core.d.ts like fill<S extends T>(value: S, start?: number, end?: number): S[];, then

const foo: number[] = new Array(3).fill("foo");
// error: "Type 'string[]' is not assignable to type 'number[]'."
// because `fill` is resolved as `fill<string>(value: string, .....): string[]`

const bar = new Array(3).fill(0); // `bar` is resolved as `number[]`
bar.fill("bar"); // error: "Type '"bar"' is not assignable to type 'number'."

const baz: (number|string)[] = new Array(3).fill(0); // accepted.
baz.fill("baz"); // accepted.

Option B. Array Constructor never return implicit any The problem is that Array constructor returns any[] implicitly. The first code is accepted even with --strict or any other options. It means we always have to care about “Array constructor returns any[]”.

Something like #26188 could solve this issue.

Playground Link: https://www.typescriptlang.org/play/#src=const foo%3A number[] %3D new Array(3).fill(“foo”)%3B

Related Issues: #29604

[MicahZoltu]

What is the reason for the Array constructor not returning an unknown[]? I’m a big fan of purging any from all of the TypeScript standard libraries, perhaps if it is too big of a change it can be added to the 4.0 todo list?

[legraphista]

@markrian The compiler and its options aren’t the problem here, it’s this line: https://github.com/microsoft/TypeScript/blob/05d59a1f1a408968aab7ae5dda9eef5881643cf7/lib/lib.es5.d.ts#L1375

form the array constructor definitions https://github.com/microsoft/TypeScript/blob/05d59a1f1a408968aab7ae5dda9eef5881643cf7/lib/lib.es5.d.ts#L1374-L1383

if you don’t specify a type, it implicitly uses the constructor which has any explicitly used.

I fear that it is too late for this to change as it will introduce many breaking changes in existing codebases. Maybe in typescript v4? We’ll see…