Rule: Don't 'in' check AST nodes?

[JoshuaKGoldberg]

From typescript-eslint/typescript-eslint#8226:

Suggestion

Sometimes folks will use an 'in' property check in a rule to narrow the type of a node:

declare const node: ts.BinaryExpression | ts.ConditionalExpression;

if ('condition' in node) {
  node
  // ^? const node: ts.ConditionalExpression
}

I've been advising to prefer using less "dynamic" logical gates: existing type predicate functions, direct .kind/.type checks, and/or having different functions for different node selectors.

declare const node: ts.BinaryExpression | ts.ConditionalExpression;

// or:
// if (node.kind === ts.SyntaxKind.ConditionalExpression) {
if (ts.isConditionalExpression(condition)) {
  node
  // ^? const node: ts.ConditionalExpression
}

My reasoning has been that "dynamic" logical gates, beyond being a potential deoptimization in JS perf, are susceptible to the types changing unexpectedly. E.g. a new version of TypeScript might add a new node type to a built-in union that adds a case previously meant to be caught by the rule.

My understanding of in checks is that they're generally reserved for cases where the more "static" ways of checking aren't available.

Shall we encode this as a lint rule?

Is this generally sound advice for lint rules altogether?

[aladdin-add]

I recognize its value, but it seems to be the type of information needed?

Otherwise there could be a lot of false positives (if obj is not an ast node):

if( key in obj) {....}

[JoshuaKGoldberg]

Yeah, this would have to be a typescript-eslint typed rule (typescript-eslint > Custom Rules > Typed Rules) to be useful in non-trivial cases.

[aladdin-add]

Currently no similar rules are included in this plugin, but I think it's possible, as long as it doesn't affect js users.
/ cc @bmish, would like to see your thoughts. 😄

[bmish]

It sounds useful to me.

Is this different from this one?

• Rule request: don't use the in operator on nodes #326

[JoshuaKGoldberg]

🤦 dear heavens. Sorry haha. I swear I did search for duplicates!