Bug: [prefer-for-of] triggered for loops that can not be converted

[yohny]

Before You File a Bug Report Please Confirm You Have Done The Following...

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have searched for related issues and found none that matched my issue.
• I have read the FAQ and my problem is not listed.

Playground Link

https://typescript-eslint.io/play/#ts=5.4.5&fileType=.ts&code=MYewdgzgLgBADgQwE4QKZJgXhmVB3GAEQHkBZABWTSQAoBKAbgFgAoUSWRFVAEy3iroAdFzQAxJCAC2AZShIAlmADmNAOQRpqGAA8pAGzUAaGGqiodUAPR7DjVu2gwkqCAFd9UAHIger-qK8QsqoUACi%2BqhSqGBQEABCAJ4AKgjKXgjR6prRAPpgvqhq9iysVlYwAAJQiXCuwIpwUAC0rvpK1nAuAGbozd0gSM0g3TCgUnD6CEoQMFAAFgiwAEao%2BiAE6yBwMBDzIB58q86okwjAvDDLiTADQyOsdzA0kbAK-AAMDDDvADwn7k8Pj8ECEkRUC2%2BCgA1NC6DAAN6sGAwcq7KBubrdVgAX1YZQqyzcsAWSxgPBA-gKsFQADcYmNpHAFJErqhgAg3GgYAAJZKkAAyAGEQPpIsAoApwMRRhSqSBYIt6TAANoyRJSZaioQKcxIJaDAC6j0Gz0csCQYBgIwBHm8hQg8KRLBRaOgmOxLDxpRYQA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Y6RAM0Wlq4D2fAV3RRe0IZHBgAviFlA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eFYDArugDTg2SrIARqwDavGlHQkATAAZp0yDwm0AJgkjiwAXV4BfEHqA&tokens=false

Repro Code

const parser = new DOMParser();
const parsed = parser.parseFromString('some xml', 'text/xml');
const resultNodes = parsed.getElementsByTagName('some_node');

// @typescript-eslint/prefer-for-of complains that below loop should be replaced by for-of
for (let i = 0; i < resultNodes.length; i++) {
  // stuff
}

// but that does not even compile because HTMLCollectionOf does not have [Symbol.iterator]
for (const rn of resultNodes) {
  // stuff
}

ESLint Config

module.exports = {
  parser: "@typescript-eslint/parser",
  rules: {
     "@typescript-eslint/prefer-for-of": "error"
  },
};

tsconfig

{
  "compilerOptions": {
    "strict": true,
    "lib": [
      "es2022",
      "dom"
    ]
  }
}

Expected Result

The rule suggesting the conversion to for-of loop should not trigger in this case

Actual Result

Rule triggers, suggesting uncompilable conversion

Additional Info

No response

[kirkwaiblinger]

I'm curious - are you able to use dom.iterable? I would think any es2022 target should support it, but I've never been totally certain on when you can and can't use dom.iterable/dom.asynciterable.

[kirkwaiblinger]

Forgot to include playground link with dom.iterable added to the config. It will get rid of the TS complaint for you 🙂 so the question is just whether you plan to run code in an environment that would support it

[Josh-Cena]

This rule is ported from tslint. It's one of the few rules that (a) doesn't handle TS specific syntax (b) doesn't use type information to check JS patterns. I think this rule ought to use type information because there could be many APIs that are indexable but not iterable—I've always been surprised that this rule false-positives so much but people have never complained.

[bradzacher]

there could be many APIs that are indexable but not iterable

Theoretically - yes.
In reality - no.

IIRC the rule looks for an indexer using the loop variable and the loop variable being incremented and the loop variable compared against a .length property.

So you need someone to build an API that exposes both contiguous numeric properties AND a .length property.
In reality this doesn't happen because ofc people just use an array.

The DOM pseudo-arrays are the only example I know of and that happens purely because legacy reasons.