How to check eslint errors

Run npx eslint . in the project root. Each problem prints as file, line, column, severity, message and rule id, and the run ends with a count. ESLint exits 1 when it found at least one error and 0 when it found only warnings, which is the part that surprises people reading a CI log.

Why check this

A lint run belongs in the pull request gate, beside the type check, and in CI on every push. It catches the defects a type checker has no opinion about: a comparison that coerces, a variable assigned and never read, a promise nobody awaited.

The failure it prevents is a rule that has been shouting for months into a log nobody reads. A warning costs nothing at merge time and exits 0, so a team can carry hundreds of them while believing the gate is closed. This procedure separates what ESLint reports from what ESLint enforces.

Prerequisites

• Node 18 or later. This page used Node 22.23.2.
• ESLint as a dev dependency. Version 9 and later read a flat config file, eslint.config.js, and ignore .eslintrc. The configuration reference describes the format.
• The project below, which produced every block on this page. Three source files and a config that turns no-console into a warning and eqeqeq into an error.

// eslint.config.js
import js from '@eslint/js';

export default [
  js.configs.recommended,
  {
    languageOptions: { globals: { console: 'readonly' } },
    rules: {
      'no-console': 'warn',
      eqeqeq: 'error',
    },
  },
];

// src/cart.js  two errors
export function total(lines) {
  const rate = 1.2;
  let sum = 0;
  for (const line of lines) {
    if (line.qty == '0') continue;
    sum += line.price * line.qty;
  }
  return sum;
}

// src/report.js  one warning, no errors
export function report(rows) {
  console.log('rows', rows.length);
  return rows.length;
}

// src/parse.ts  broken, and the config does not match it
export function parse(raw: string): number {
  return undefinedHelper(raw);
}

Steps

1. 1

   Step 1.

   Read the ESLint version the project resolves to. The rule set and the config format both depend on it.

   npx eslint --version
   

   v10.10.0

2. 2

   Step 2.

   Lint the project and read the exit code in the same line.

   npx eslint .; echo "exit=$?"
   

   D:\how2check\scratch\lintq\src\cart.js
   2:9   error  'rate' is assigned a value but never used  no-unused-vars
   5:18  error  Expected '===' and instead saw '=='        eqeqeq
   
   D:\how2check\scratch\lintq\src\report.js
   2:3  warning  Unexpected console statement  no-console
   
   ✖ 3 problems (2 errors, 1 warning)
   
   exit=1

   The last column is the rule id. It is the only part worth quoting in a ticket, because the message text changes between versions.

3. 3

   Step 3.

   Lint the file that has a warning and no error, and read the exit code again.

   npx eslint src/report.js; echo "exit=$?"
   

   D:\how2check\scratch\lintq\src\report.js
   2:3  warning  Unexpected console statement  no-console
   
   ✖ 1 problem (0 errors, 1 warning)
   
   exit=0

   ESLint printed a problem and reported success. A CI step that reads only the status treats this run as clean.

4. 4

   Step 4.

   Make warnings count.

   npx eslint src/report.js --max-warnings 0; echo "exit=$?"
   

   D:\how2check\scratch\lintq\src\report.js
   2:3  warning  Unexpected console statement  no-console
   
   ✖ 1 problem (0 errors, 1 warning)
   
   ESLint found too many warnings (maximum: 0).
   exit=1

   Same output, same warning, different status. This is the flag that turns a report into a gate.

5. 5

   Step 5.

   List the files the run actually read, rather than the files it reported on.

   npx eslint . --format json | node -e "let s='';process.stdin.on('data',d=>s+=d).on('end',()=>JSON.parse(s).forEach(r=>console.log(r.filePath)))"
   

   D:\how2check\scratch\lintq\eslint.config.js
   D:\how2check\scratch\lintq\src\cart.js
   D:\how2check\scratch\lintq\src\report.js

   Three files. src/parse.ts is not among them, and step 2 said nothing about it.

6. 6

   Step 6.

   Point ESLint straight at the file it skipped.

   npx eslint src/parse.ts; echo "exit=$?"
   

   D:\how2check\scratch\lintq\src\parse.ts
   0:0  warning  File ignored because no matching configuration was supplied
   
   ✖ 1 problem (0 errors, 1 warning)
   
   exit=0

   Naming the file does not lint it. The refusal arrives as a warning at line 0, and the run still exits 0.

How to read the result

| What you see | What it means | What to do | | --- | --- | --- | | No output at all, exit 0 | Every matched file passed every enabled rule | Confirm the matched file list with step 5 before believing it. | | error rows, exit 1 | At least one rule set to error fired | Fix them. --fix handles the mechanical ones. | | Only warning rows, exit 0 | Rules are reporting, nothing is enforced | Add --max-warnings 0 to the CI command, or raise the rules to error. | | File ignored because no matching configuration was supplied | No config entry matches that extension | Add a files entry and a parser for it. Until then the file is unlinted. | | exit 2 with a stack trace | The config itself failed to load | Read the module name in the error. A missing config dependency looks nothing like a lint failure. |

Common mistakes

What to check next

• How to check typescript errors: the type check that runs beside this one, with an exit code of its own.
• How to check prettier formatting: formatting is a separate gate, and mixing it into lint rules slows both.
• How to check installed npm package version: resolves the version question in step 1 when two machines disagree.
• How to check outdated npm packages: a major ESLint upgrade changes the config format and the default rule set.
• Pull request checklist: where this check sits in the gate.

FAQ

How do I check my ESLint version?

npx eslint --version prints the version the project resolves, v10.10.0 here. Prefer it to a global eslint --version, which can report a different install. The config format changed at version 9, so the number decides whether eslint.config.js or .eslintrc is read.

Why does ESLint exit 0 when it printed problems?

Warnings do not fail the run. Only a rule set to error moves the exit code to 1. Add --max-warnings 0 to make the count fail the command, as step 4 shows.

How do I see only the errors?

npx eslint . --quiet suppresses the warning rows and reports errors alone. Combined with --max-warnings 0 the warnings are still counted: on src/report.js that pair prints no rows and exits 1.

Does ESLint check TypeScript files?

Not with the config above. TypeScript needs typescript-eslint, a parser entry and a files pattern that includes the extension. Without them the files are skipped silently, which step 5 makes visible.

Can ESLint fix what it reports?

Some of them. A fixable rule does not make every violation fixable: --fix-dry-run --format json on src/cart.js reports fixableErrorCount: 0, because rewriting line.qty == '0' as === would change the result. Run the check again after a fix pass.

Verified