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Unable to call map on array of variable depth #35045
Description
I'm trying to write a function which transforms the coordinates of any type of GeoJSON geometry. The coordinates of a Geometry can be either number[] (a Point), number[][] (a LineString), number[][][] (a Polygon) or number[][][][] (a MultiPolygon). I have a recursive implementation which works but does not type check.
TypeScript Version: 3.7.2
Search Terms:
- 2349
Code
type Position = number[];
function isNestedArray<T extends any[]>(x: T | T[]): x is T[] {
return Array.isArray(x[0]);
}
export function transformCoordArray(
coords: Position | Position[] | Position[][],
fn: (pt: Position) => Position
): any[] {
if (isNestedArray(coords)) {
coords; // type is Position[] | Position[][]
return coords.map(c => transformCoordArray(c, fn));
// ~~~ This expression is not callable.
} else {
coords; // type is Position
return fn(coords);
}
}The full error message is:
This expression is not callable.
Each member of the union type '
(<U>(callbackfn: (value: Position, index: number, array: Position[]) => U, thisArg?: any) => U[])
| (<U>(callbackfn: (value: Position[], index: number, array: Position[][]) => U, thisArg?: any) => U[])
' has signatures, but none of those signatures are compatible with each other.(2349)
The call is valid because c's type should be Position | Position[], which is assignable to Position | Position[] | Position[][].
Interestingly, I can make the error go away by assigning to a new variable which has a broader type ((Position | Position[])[]):
(playground)
export function transformCoordArray(
coords: Position | Position[] | Position[][],
fn: (pt: Position) => Position
): any[] {
if (isNestedArray(coords)) {
coords; // type is Position[] | Position[][]
const c2: (Position | Position[])[] = coords;
return c2.map(c => transformCoordArray(c, fn)); // ok
} else {
coords; // type is Position
return fn(coords);
}
}Expected behavior:
No error.
Actual behavior:
The given error.
Playground Link: playground
Related Issues:
Activity
jcalz commentedon Nov 12, 2019
Duplicate of #7294 (see comment). Related to #29011.
Mentioned in documentation as caveats for improved behavior for calling union types:
thorn0 commentedon Nov 12, 2019
While methods like
pushare obviously problematic fornumber[] | string[], it looks safe to allow methods likemaporeveryforreadonly number[] | readonly string[]. Has this (special treatment of read-only arrays when merging union signatures) already been discussed in some issue?thorn0 commentedon Nov 15, 2019
We are trying to gradually introduce type checking in the Prettier project and facing this issue.
Prettier works with ASTs that come from different parsers.
E.g. suppose we have an object
nodeof typeimport('estree').Node | import('@typescript-eslint/typescript-estree').TSESTree.Nodeand code like this:Even though
node.propertiesare correctly inferred to be of typeObjectLiteralElementLike[] | Property[](because bothestreeand@typescript-eslint/typescript-estreedefineObjectExpression, not exactly compatible though),pin theeverycallback is of typeany. 😢hinell commentedon Nov 16, 2019
I think it's inherently bad idea to rely on compiler to figure out the output of the
transformCoordArray()meanwhilecoords: ...argument varies.I think much better way either to create overloads (checkout playground):
or assert
coords:jcalz commentedon May 27, 2021
Fixed by #42620 I think?
danvk commentedon May 27, 2021
Yep! The error in the example in the original issue goes away with 4.3: playground