Suggestion: Add signature for Object.keys() to remove "element implicitly has an 'any'" error

[nitzantomer]

When using noImplicitAny the following code:

interface Point3D {
    x: number;
    y: number;
    z: number;
}

let p1: Point3D = { x: 0, y: 0, z: 0 };
let p2 = {} as Point3D;
Object.keys(p1).forEach(key => p2[key] = p1[key]);

Results with the following error:

error TS7017: Element implicitly has an 'any' type because type 'Point3D' has no index signature

There are ways to avoid it:

1. Use suppressImplicitAnyIndexErrors
2. Adding an index signature to the interface
3. Cast p2 and p1 to any
4. Don't use noImplicitAny

There's another option:

type keys = keyof Point3D;
(Object.keys(p1) as keys[]).forEach(key => p2[key] = p1[key]);

That can be supported by adding a signature for Object.keys in the lib.d.ts:

interface ObjectConstructor {
    ...
    keys(o: any): string[];
    keys<T>(o: T): (keyof T)[];
}

Then the original code would work without the need for suppressImplicitAnyIndexErrors.

[mhegazy]

Just to be clear, the error is correct to me. Types in typescript are not final, so there is no guarantees that p1 does not have more keys.. consider:

function fix(p: Point3D) {
    Object.keys(p).forEach(key => p[key] = p[key].toFixed()); // p[comment] does not have 
}

let p1 = { x: 0, y: 0, z: 0, comment: "my 3d point" };
fix(p1);

If you are sure, then cast before access. Alternatively, you can define an index signature on your Point3D.

[KostyaTretyak]

Alternatively, you can define an index signature on your Point3D

@mhegazy, but if we define this way, its not works:

type WithSignaturePoint3D = { [P in 'x' | 'y' | 'z']: number };

function fix(p: WithSignaturePoint3D)
{
  // Why here throw error?
  Object.keys(p).forEach(key => console.log(p[key]) );
}

[mhegazy]

{ [P in 'x' | 'y' | 'z']: number } === { x: number; y: number; z: number }. So not sure i understand the question.

[KostyaTretyak]

@mhegazy, you say that

Alternatively, you can define an index signature on your Point3D

I define WithSignaturePoint3D with index signature, but TypeScript still throw error.