Incorrect type error when assigning to an index in a `{}[]`-typed field where the index is any-typed and field is defined in a base class in a JS file

[peetklecha]

Steps to reproduce

// dep.js

export class BaseClass {
    constructor() {
        this.map = [];
        this.map.push(function(){});
    }
}

// main.ts

import { BaseClass } from "./dep.js";

declare const key: any;

class Item extends BaseClass {
    constructor() {
        super();
        this.map[key] = {};
//      ~~~~~~~~~~~~~
// Type '{}' is not assignable to type '{}'. Two different types with this name exist, but they are unrelated.
    }
}

Behavior with typescript@5.9

No error is raised.

Behavior with tsgo

The line this.map[key] = {} raises error 2719; or if any other value is used (e.g., this.map[key] = 7) error 2322. I have noticed other cases where tsgo seems not quite do quite such aggressive type inference on arrays defined in JS files but in this case it does seemingly still identify map as having type {}[].

[ahejlsberg]

This error is intended.

The {} in the inferred {}[] type is actually the internal anyFunctionType, a wildcard type used during type inference. It's somewhat suspicious to have that type escape, but probably not worth changing. tsgo has stricter rules for anyFunctionType and treats it as a subtype of any other object type (and therefore nothing is assignable to it).

With microsoft/TypeScript#62243, your example errors in TS 6.0 because the inferred type of the array becomes (() => void)[]. That will eventually be the behavior in tsgo also.