Add no-never-initialized-let rule

[jtbandes]

Changelog

Added @foxglove/no-never-initialized-let (enabled by default).

Docs

Added to readme

Description

Adds a rule to fill a gap when init-declarations is not enabled. Related discussions: eslint/eslint#19581 & microsoft/TypeScript#61496

Copying from readme:

@foxglove/no-never-initialized-let

Disallow variable declarations that use let but have no intitial value and are never assigned. These variables will always be undefined and are likely a programmer error.

The builtin prefer-const rule doesn't flag these because they lack an initializer. Otherwise, they could be flagged by init-declarations, but this rule is mostly stylistic and has some implications for TypeScript type inference & refinement. (See eslint/eslint#19581 & microsoft/TypeScript#61496 for more discussion.)

Examples of incorrect code for this rule:

let prevX;
let prevY;
if (x !== prevX) {
  prevX = x;
}
if (y !== prevY) {
  prevX = x; // typo, should have been Y
}

Examples of correct code for this rule:

let prevX;
let prevY;
if (x !== prevX) {
  prevX = x;
}
if (y !== prevY) {
  prevY = y;
}

[drdelambre]

not against it on it's face. hard to gauge impact when it's in a different repo

[jtbandes]

FWIW I ran it against our main repo and it did not flag any additional cases. (The one case that launched this journey has already been fixed)

So I guess not very high impact, but I think this basically always indicates a programmer error 🤷