Typescript: Type guards in Array.prototype.every

[kylewlacy]

TypeScript Version: 2.2.2

Code

function isNumber(x: any): x is number { return typeof(x) === "number"; }

let numbersOrStrings: (number | string)[] = [1, 2, 3, 4];

if(numbersOrStrings.every(isNumber)) {
    let _numbers: number[] = numbersOrStrings;
    // ...
}

Expected behavior:

When every member of numberOfStrings matches the isNumber type guard, the whole array should be inferred to be numberOfStrings: number[]; so, the above code should compile without error.

Actual behavior:

The above fails to compile, with the following error:

error TS2322: Type '(string | number)[]' is not assignable to type 'number[]'.

---

This issue is similar to #7657, and it can be solved by adding something like the following to lib.d.ts:

interface Array<T> {
    every<U extends T>(pred: (a: T) => a is U): this is U[];
}

(or, the above can be added locally to a project as a temporary workaround)

[vkurchatkin]

This issue is similar to #7657,

It's not the same, though. filter creates a new array, every doesn't. Assigning (string | number)[] to a number[] is unsafe, because a string can be pushed at any moment.

[kylewlacy]

@vkurchatkin Would adding the above declaration for ReadonlyArray<T> be safe, then?

[vkurchatkin]

@kylewlacy yes, I think so

[mhegazy]

related to https://github.com/Microsoft/TypeScript/issues/5101

[mhegazy]

16069 now tracks inferring the type guard automatically.