[no-unsafe-assignment] fails to infer the type from class properties

[photonstorm]

Repro

import { IEventInstance } from './IEventInstance';

export class EventEmitter
{
    events: Map<string, Set<IEventInstance>>;

    constructor ()
    {
        this.events = new Map();
    }
}

Expected Result

The no-unsafe-assignment rule is flagging the creation of the Map in the constructor as being unsafe, claiming it can accept any types, even though the type is enforced by the events property in the class.

Actual Result

I see no logical reason why this should be considered an unsafe assignment when the type is clearly enforced by the property. TypeScript itself prevents incorrect insertion of values into this Map.

Additional Info

In order to stop the rule complaining, the following works:

import { IEventInstance } from './IEventInstance';

export class EventEmitter
{
    events: Map<string, Set<IEventInstance>>;

    constructor ()
    {
        this.events = new Map<string, Set<IEventInstance>>();
    }
}

However, this is just duplicating the type already defined.

Versions

package	version
@typescript-eslint/eslint-plugin	3.0.1
@typescript-eslint/parser	3.0.1
TypeScript	3.9.3
ESLint	7.1.0
node	10.16.0
npm	6.9.0

[bradzacher]

Yeah I ran into this when attempting to introduce the rule into this codebase.

For some reason TS doesn't automatically infer the type of the RHS if it's new Map().
This works 100% of the time for new Set(), so it's unique to map.

I think it's due to how the Map constructor is defined.
https://github.com/microsoft/TypeScript/blob/6ac6a0410988e9f400c9153ac2b5fe9419849bf3/src/lib/es2015.collection.d.ts#L11-L16

interface MapConstructor {
    new(): Map<any, any>;
    new<K, V>(entries?: readonly (readonly [K, V])[] | null): Map<K, V>;
    readonly prototype: Map<any, any>;
}
declare var Map: MapConstructor;

Compared to Set:
https://github.com/microsoft/TypeScript/blob/6ac6a0410988e9f400c9153ac2b5fe9419849bf3/src/lib/es2015.collection.d.ts#L47-L51

interface SetConstructor {
    new <T = any>(values?: readonly T[] | null): Set<T>;
    readonly prototype: Set<any>;
}
declare var Set: SetConstructor;

Note that Map has an empty constructor which defaults to anys, and a parameterised constructor that doesn't.
OTOH Set only has a parameterised constructor, but its generic type has a default.

Unsure if it's a "bug" in TS or not. Either way, it's something that will have to be special cased in the rule unfortunately.

[bradzacher]

Doing some research, it looks like this is intentional.
Clicking through the history for the lib def, you find microsoft/TypeScript#25374 which did exactly what we want!

However because Map has multiple type parameters, this caused weirdness with extending Map (microsoft/TypeScript#27951), which then lead to just the change to just the Map constructor being reverted (microsoft/TypeScript#28052).

So the map constructor being like this is completely intentional, which means we'll have to add a special case for new Map().

[photonstorm]

Thanks for the quick update. Sounds like this is quite the rabbit hole! I've sadly found I have had to disable all of the unsafe rules for my codebase because they seem to conflict with TS in subtle ways. I don't envy you having to work with such a moving feast as this! Will keep an eye out at the Change Log.

[bradzacher]

they seem to conflict with TS in subtle ways

How so? If there are problems with the rule, please report them so we can look into getting them fixed!

[Raynos]

I ran into this issues today as well.

Would be lovely if Map can be special cased in @typescript-eslint because it's builtin into typescript stdlib definitions.

If I implement my own custom Generic MapLike and have an any signature in my own class definition then it's a "bug" in my constructor because I shouldn't use any.

Fixing TypeScript standard library for ES6 / etc to remove any is a large task ... so special casing Map in @typescript-eslint would be very practical.

[Raynos]

Here's an example workaround for JSDoc.

class Foo {
  constructor () {
    // https://github.com/typescript-eslint/typescript-eslint/issues/1943
    /* eslint-disable @typescript-eslint/no-unsafe-assignment */
    /** @type {Map<string, FunctionConfiguration[]>} */
    this._profiles = /** @type {Map<string, FunctionConfiguration[]>} */ (new Map())
    /* eslint-enable @typescript-eslint/no-unsafe-assignment */
  }
}

JSdoc does not support new Map<string, string> but does support casts ... I opened an issue in TypeScript for the lacking JSDoc functionality in function calls ( microsoft/TypeScript#38876 ).