Question: implicit "any" from "yield" warning?

[Vanuan]

Is it feasible to add a warning for yield result to be assigned to non-typed variable or as non-typed argument?

TypeScript doesn't always infer the yield return type:

microsoft/TypeScript#26959

[bradzacher]

This will be handled more broadly by the no-unsafe-* set of rules I've been building.

• no-unsafe-call
• no-unsafe-member-access
• no-unsafe-return
• And the not yet merged no-unsafe-assignment feat(eslint-plugin): add rule no-unsafe-assignment #1694

[Vanuan]

Awesome! Would it also handle unsafe array indexing?

[bradzacher]

Yes, no-unsafe-member-access handles that as well.

[Vanuan]

Example:

const a: number[] = [];
const b: number | null = a[0] || null;
b.toFixed(); // runtime error, no type error

[Vanuan]

More complex example:

interface SimpleType { 
    id: string;
}
interface ComplexType {
    arrayField: SimpleType[];
}
const myObject = {
    arrayField: [],
}
const a: SimpleType[] = myObject.arrayField;
const b: SimpleType | null = a[0] || null;
b.id; // runtime error, no type error, should be Object is possibly 'null'.

[Vanuan]

Well, it looks like the latter is a bug in TypeScript. The type error is working for primitive type, but not for user-defined types.

Closing the issue as it's a different question.

Thank you!

[bradzacher]

Ah that's what you meant by unsafe indexing.
No, there's no rule for that. That sort of thing ideally has to be built into typescript itself, as it requires control-flow analysis of the variable.

You could also encode this in the types, and ban member expressions on non-tuple arrays.

function hasLength<T>(n: 1, arr: T[]): arr is [T, ...T[]];
function hasLength<T>(n: 2, arr: T[]): arr is [T, T, ...T[]];
function hasLength<T>(n: 3, arr: T[]): arr is [T, T, T, ...T[]];
// ....
function hasLength<T>(n: number, arr: T[]): arr is T[] {
    return arr.length === n;
};

declare const x: number[];

const y = hasLength(2, x); // typeof y === [number, number, ...number[]];

x[0] // hypothetical-lint-rule-error: No array index access, use hasLength to validate the length first
y[0] // no error, as it's provably safe