8000 Bug: @typescript-eslint/no-unnecessary-type-assertion False positive on non-null assertion after an implicitly-any variable gets initialised inside a conditional or loop · Issue #11054 · typescript-eslint/typescript-eslint · GitHub
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Bug: @typescript-eslint/no-unnecessary-type-assertion False positive on non-null assertion after an implicitly-any variable gets initialised inside a conditional or loop #11054

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ExplodingCabbage opened this issue Apr 9, 2025 · 1 comment · May be fixed by #11082
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Bug: @typescript-eslint/no-unnecessary-type-assertion False positive on non-null assertion after an implicitly-any variable gets initialised inside a conditional or loop #11054

ExplodingCabbage opened this issue Apr 9, 2025 · 1 comment · May be fixed by #11082
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accepting prs Go ahead, send a pull request that resolves this issue bug Something isn't working package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin

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@ExplodingCabbage
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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=5.8.2&fileType=.ts&code=GYVwdgxgLglg9mABAUwB4EMC2AHANsgCgEpEBvAKEUXykVQG5LEZhECBZdKACwDoAndGAAmcTMUQA%2BRAFoAjCQpUqqRAF5EAJkZUAvk37IoIfklQBCRAGpEcxvvJoseQkUZA&eslintrc=N4KABGBECmAeAu0B2ATAzpAXGA2uCUADgDYCuA5gJZKYAC8AnodGgMYBOlh8AtC8dXgB6dtFYB7ALaTkKaCh6NmPVgAsxAa3mR8AXQA0%2BSO1LEWWMMAC%2BIK0A&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eFYDAruuGAL4g9A&tokens=false

Repro Code

function example() {
  let x;
  if (Math.random() > -1) {
    x = 2;
  }
  return x! + 1;
}

example();

ESLint Config

import tseslint from 'typescript-eslint';

export default tseslint.config(
  tseslint.configs.recommendedTypeChecked,
  {
    languageOptions: {
      parserOptions: {
        projectService: true,
        tsconfigRootDir: import.meta.dirname,
      },
    },
  },
  {
    files: ['eslint.config.mjs'],
    extends: [tseslint.configs.disableTypeChecked],
  }
);

tsconfig

{
    "include": ["**/*.ts"],
    "compilerOptions": {
        "strict": true,
        
    }
}

Expected Result

No errors, since the non-null assertion (!) is necessary here; without it, tsc will refuse to compile the code, complaining that `'x' is possibly 'undefined'!

Actual Result

6:10  error  This assertion is unnecessary since it does not change the type of the expression  @typescript-eslint/no-unnecessary-type-assertion

Additional Info

Things that seem necessary to produce the error above:

  • x is not explicitly typed when declared (so is implicitly any)
  • the initial assignment of x takes place inside a conditional or a loop, and TypeScript therefore can't tell, without the non-null assertion, that the initial assignment is guaranteed to take place
  • strict: true is enabled

The example code above is of course contrived, but a more realistic example where this would in fact be totally reasonable code would be one where, instead of an if (...) {...} that's guaranteed to run, the assignment takes place within a loop that's guaranteed to run (and x gets used to store some sort of information about the final iteration of the loop that gets used after the loop finishes).
@ExplodingCabbage ExplodingCabbage added bug Something isn't working package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin triage Waiting for team members to take a look labels Apr 9, 2025
@ronami
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ronami commented Apr 9, 2025

Interesting, seems like a missed edge case in isPossiblyUsedBeforeAssigned.

@bradzacher bradzacher added accepting prs Go ahead, send a pull request that resolves this issue and removed triage Waiting for team members to take a look labels Apr 10, 2025
@mdm317 mdm317 linked a pull request Apr 16, 2025 that will close this issue
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@cainlevy cainlevy mentioned this issue May 22, 2025
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