Custom Formatters

Custom formatters allow you to control how ESLint displays linting results. Whether you need JSON for CI/CD integration, HTML reports for dashboards, or custom formats for specific tools, formatters make it possible.

When to Create a Formatter

Create a custom formatter when you need to:

Generate reports for CI/CD systems
Create HTML dashboards or visualizations
Integrate with IDEs or editors
Export to specific file formats (XML, CSV, JUnit)
Apply custom styling or branding
Aggregate results across multiple runs

Built-in Formatters:ESLint includes several built-in formatters: stylish, compact, json, junit, html, and more. Use these as references for creating your own.

Formatter Structure

A formatter is a function that receives results and returns a string:

module.exports = function(results, context) {
  // Process results
  return "formatted output string";
};

Synchronous Formatter

my-formatter.js

module.exports = function(results, context) {
  let output = "";
  
  results.forEach(result => {
    output += `${result.filePath}\n`;
    
    result.messages.forEach(msg => {
      output += `  ${msg.line}:${msg.column} ${msg.message}\n`;
    });
  });
  
  return output;
};

Async Formatter

my-async-formatter.js

module.exports = async function(results, context) {
  // Perform async operations
  await sendToAPI(results);
  
  // Format and return
  return JSON.stringify(results, null, 2);
};

The Results Parameter

The results parameter is an array of LintResult objects:

[
  {
    filePath: "/path/to/file.js",
    messages: [
      {
        ruleId: "no-unused-vars",
        severity: 2,
        message: "'x' is defined but never used.",
        line: 2,
        column: 7,
        endLine: 2,
        endColumn: 8,
        nodeType: "Identifier"
      }
    ],
    errorCount: 1,
    warningCount: 0,
    fixableErrorCount: 0,
    fixableWarningCount: 0,
    source: "const x = 1;\nconsole.log('hello');\n"
  }
]

LintResult Properties

filePath

string

Absolute path to the linted file

messages

LintMessage[]

Array of lint messages for this file

errorCount

number

Number of errors (severity 2)

warningCount

number

Number of warnings (severity 1)

fixableErrorCount

number

Number of errors that can be auto-fixed

fixableWarningCount

number

Number of warnings that can be auto-fixed

source

string

Source code of the file (may be omitted)

suppressedMessages

LintMessage[]

Messages that were suppressed by inline comments

LintMessage Properties

ruleId

string | null

ID of the rule that generated the message

severity

0 | 1 | 2

Message severity: 0 (off), 1 (warn), 2 (error)

message

string

Human-readable error message

line

number

1-based line number where the issue occurs

column

number

1-based column number where the issue occurs

endLine

number

1-based line number where the issue ends

endColumn

number

1-based column number where the issue ends

fix

object

Auto-fix information (if available)

{
  range: [0, 5],
  text: "const"
}

suggestions

array

Alternative fixes that require manual approval

The Context Parameter

The context object provides additional information:

{
  cwd: "/project/root",
  color: true,  // or false, or undefined
  maxWarningsExceeded: {
    maxWarnings: 5,
    foundWarnings: 6
  },
  rulesMeta: {
    "no-unused-vars": {
      type: "problem",
      docs: {
        description: "Disallow unused variables",
        recommended: true,
        url: "https://eslint.org/docs/rules/no-unused-vars"
      },
      fixable: "code",
      messages: {
        unusedVar: "'{{varName}}' is defined but never used."
      }
    }
  }
}

cwd

string

Current working directory

color

boolean | undefined

Whether color output was requested (--color or --no-color)

maxWarningsExceeded

object | undefined

Present if --max-warnings was exceeded

maxWarnings: The limit that was set
foundWarnings: Actual number of warnings

rulesMeta

object

Metadata for all rules that were run

Example: Summary Formatter

Display only error and warning counts:

module.exports = function(results, context) {
  // Calculate totals
  const summary = results.reduce(
    (acc, result) => ({
      errors: acc.errors + result.errorCount,
      warnings: acc.warnings + result.warningCount
    }),
    { errors: 0, warnings: 0 }
  );
  
  // Format output
  if (summary.errors > 0 || summary.warnings > 0) {
    return `Errors: ${summary.errors}, Warnings: ${summary.warnings}\n`;
  }
  
  return "No problems found!\n";
};

Usage:

eslint -f ./my-formatter.js src/

Output:

Errors: 2, Warnings: 4

Example: Detailed Formatter

Include file paths and rule information:

module.exports = function(results, context) {
  const output = [];
  
  results.forEach(result => {
    if (result.messages.length === 0) return;
    
    output.push(`\nFile: ${result.filePath}`);
    
    result.messages.forEach(msg => {
      const severity = msg.severity === 2 ? "error" : "warning";
      const ruleInfo = context.rulesMeta[msg.ruleId];
      const ruleUrl = ruleInfo?.docs?.url || "";
      
      output.push(
        `  ${msg.line}:${msg.column} ${severity} ${msg.message} (${msg.ruleId})${
          ruleUrl ? ` ${ruleUrl}` : ""
        }`
      );
    });
  });
  
  return output.join("\n") + "\n";
};

Output:

File: /project/src/app.js
  2:7 error 'x' is defined but never used. (no-unused-vars) https://eslint.org/docs/rules/no-unused-vars
  5:1 warning Unexpected console statement. (no-console) https://eslint.org/docs/rules/no-console

Example: JSON Formatter

Format as JSON for tool integration:

module.exports = function(results, context) {
  // ESLint's built-in JSON formatter
  return JSON.stringify(results, null, 2);
};

Example: HTML Report

Generate an HTML dashboard:

module.exports = function(results, context) {
  const totalErrors = results.reduce((sum, r) => sum + r.errorCount, 0);
  const totalWarnings = results.reduce((sum, r) => sum + r.warningCount, 0);
  
  const html = `
<!DOCTYPE html>
<html>
<head>
  <title>ESLint Report</title>
  <style>
    body { font-family: Arial, sans-serif; margin: 20px; }
    .summary { padding: 20px; background: #f5f5f5; }
    .error { color: #d32f2f; }
    .warning { color: #f57c00; }
    .file { margin: 20px 0; border: 1px solid #ddd; padding: 15px; }
    .message { margin: 10px 0; padding: 10px; background: #fff; }
  </style>
</head>
<body>
  <h1>ESLint Report</h1>
  
  <div class="summary">
    <h2>Summary</h2>
    <p class="error">Errors: ${totalErrors}</p>
    <p class="warning">Warnings: ${totalWarnings}</p>
  </div>
  
  ${results.map(result => `
    <div class="file">
      <h3>${result.filePath}</h3>
      ${result.messages.map(msg => `
        <div class="message ${msg.severity === 2 ? 'error' : 'warning'}">
          <strong>Line ${msg.line}:${msg.column}</strong> - ${msg.message}
          <br><small>Rule: ${msg.ruleId}</small>
        </div>
      `).join('')}
    </div>
  `).join('')}
</body>
</html>
  `;
  
  return html;
};

Example: JUnit XML

Generate JUnit-compatible XML for CI systems:

function xmlEscape(str) {
  return str
    .replace(/&/g, "&amp;")
    .replace(/</g, "&lt;")
    .replace(/>/g, "&gt;")
    .replace(/"/g, "&quot;")
    .replace(/'/g, "&apos;");
}

module.exports = function(results, context) {
  let output = '<?xml version="1.0" encoding="UTF-8"?>\n';
  output += '<testsuites>\n';
  
  results.forEach(result => {
    const testCount = result.messages.length;
    const failures = result.messages.filter(m => m.severity === 2).length;
    
    output += `  <testsuite name="${xmlEscape(result.filePath)}" tests="${testCount}" failures="${failures}">\n`;
    
    result.messages.forEach((msg, index) => {
      const testName = `${msg.ruleId}:${msg.line}:${msg.column}`;
      
      output += `    <testcase name="${xmlEscape(testName)}" classname="${xmlEscape(result.filePath)}">\n`;
      
      if (msg.severity === 2) {
        output += `      <failure message="${xmlEscape(msg.message)}">`;
        output += xmlEscape(`${msg.line}:${msg.column} ${msg.message}`);
        output += `</failure>\n`;
      }
      
      output += `    </testcase>\n`;
    });
    
    output += `  </testsuite>\n`;
  });
  
  output += '</testsuites>\n';
  
  return output;
};

Using Environment Variables

Customize formatter behavior with environment variables:

module.exports = function(results, context) {
  const skipWarnings = process.env.FORMATTER_SKIP_WARNINGS === "true";
  const verbose = process.env.FORMATTER_VERBOSE === "true";
  
  const summary = results.reduce(
    (acc, result) => {
      result.messages.forEach(msg => {
        const logMessage = {
          filePath: result.filePath,
          ruleId: msg.ruleId,
          message: msg.message,
          line: msg.line,
          column: msg.column
        };
        
        if (msg.severity === 1) {
          if (!skipWarnings) {
            acc.warnings.push(logMessage);
          }
        } else if (msg.severity === 2) {
          acc.errors.push(logMessage);
        }
      });
      return acc;
    },
    { errors: [], warnings: [] }
  );
  
  let output = "";
  
  [...summary.errors, ...summary.warnings].forEach(msg => {
    output += `${msg.filePath}:${msg.line}:${msg.column} ${msg.message}`;
    
    if (verbose) {
      output += ` (${msg.ruleId})`;
    }
    
    output += "\n";
  });
  
  return output;
};

Usage:

FORMATTER_SKIP_WARNINGS=true eslint -f ./my-formatter.js src/
FORMATTER_VERBOSE=true eslint -f ./my-formatter.js src/

Terminal-Friendly Output

Modern terminals support clickable links:

module.exports = function(results, context) {
  const output = [];
  
  results.forEach(result => {
    result.messages.forEach(msg => {
      // Format: file:line:column
      output.push(
        `${result.filePath}:${msg.line}:${msg.column} ` +
        `${msg.message} (${msg.ruleId})`
      );
    });
  });
  
  return output.join("\n") + "\n";
};

Output (clickable in many terminals):

/project/src/app.js:5:1 Unexpected console statement. (no-console)
/project/src/app.js:10:7 'x' is defined but never used. (no-unused-vars)

Color Support

Add colors when requested:

module.exports = function(results, context) {
  const useColor = context.color;
  
  const red = useColor ? "\x1b[31m" : "";
  const yellow = useColor ? "\x1b[33m" : "";
  const reset = useColor ? "\x1b[0m" : "";
  const bold = useColor ? "\x1b[1m" : "";
  
  const output = [];
  
  results.forEach(result => {
    if (result.messages.length === 0) return;
    
    output.push(`\n${bold}${result.filePath}${reset}`);
    
    result.messages.forEach(msg => {
      const color = msg.severity === 2 ? red : yellow;
      const level = msg.severity === 2 ? "error" : "warning";
      
      output.push(
        `  ${msg.line}:${msg.column} ` +
        `${color}${level}${reset} ` +
        `${msg.message} ` +
        `${color}(${msg.ruleId})${reset}`
      );
    });
  });
  
  return output.join("\n") + "\n";
};

Creating Your Formatter

Create Formatter File

my-formatter.js

module.exports = function(results, context) {
  // Start with a simple implementation
  const output = [];
  
  results.forEach(result => {
    output.push(result.filePath);
    
    result.messages.forEach(msg => {
      output.push(`  ${msg.line}:${msg.column} ${msg.message}`);
    });
  });
  
  return output.join("\n");
};

Test Locally

eslint -f ./my-formatter.js src/

Add Features

Enhance with:

Summary statistics
Color support
Rule metadata
Links to documentation

Package for Distribution

Create an npm package (optional):

package.json

{
  "name": "eslint-formatter-custom",
  "main": "index.js",
  "keywords": ["eslint", "eslint-formatter", "eslintformatter"]
}

Using a Custom Formatter

Local File

# Relative path (must start with .)
eslint -f ./formatters/my-formatter.js src/

# Absolute path
eslint -f /path/to/my-formatter.js src/

npm Package

# Install
npm install --save-dev eslint-formatter-custom

# Use (ESLint looks for eslint-formatter-* packages)
eslint -f custom src/

Packaging a Formatter

Create Package Structure

eslint-formatter-custom/
├── package.json
├── index.js
├── README.md
└── test/
    └── formatter.test.js

Configure package.json

{
  "name": "eslint-formatter-custom",
  "version": "1.0.0",
  "description": "Custom ESLint formatter",
  "main": "index.js",
  "keywords": [
    "eslint",
    "eslint-formatter",
    "eslintformatter"
  ],
  "peerDependencies": {
    "eslint": ">=9.0.0"
  }
}

Implement Formatter

index.js

module.exports = function(results, context) {
  // Your formatter implementation
};

Publish

npm publish

Testing Formatters

const assert = require("assert");
const formatter = require("./my-formatter");

const results = [
  {
    filePath: "/path/to/file.js",
    messages: [
      {
        ruleId: "no-unused-vars",
        severity: 2,
        message: "'x' is defined but never used.",
        line: 2,
        column: 7
      }
    ],
    errorCount: 1,
    warningCount: 0,
    fixableErrorCount: 0,
    fixableWarningCount: 0
  }
];

const context = {
  cwd: "/project",
  rulesMeta: {
    "no-unused-vars": {
      docs: {
        description: "Disallow unused variables"
      }
    }
  }
};

const output = formatter(results, context);

assert(output.includes("file.js"));
assert(output.includes("no-unused-vars"));
console.log("Tests passed!");

Advanced Patterns

Piping to External Tools

For complex formatting, use ESLint’s JSON output:

eslint -f json src/ | your-tool --process

Your tool receives:

const results = JSON.parse(stdin);
// Process and format as needed

Async Formatters

Perform asynchronous operations:

module.exports = async function(results, context) {
  // Upload to API
  await fetch("https://api.example.com/lint-results", {
    method: "POST",
    body: JSON.stringify(results)
  });
  
  // Return formatted output
  return `Results uploaded. Errors: ${totalErrors}\n`;
};

Stateful Formatters

Maintain state across runs (use with caution):

let runCount = 0;

module.exports = function(results, context) {
  runCount++;
  
  return `Run #${runCount}: ${totalErrors} errors\n`;
};

Best Practices

Follow these guidelines:

Always return a string - Even if empty
Handle empty results - Check for files with no messages
Escape special characters - Especially in HTML/XML formatters
Make output parseable - Use consistent formats
Document your format - Explain the output structure
Test with edge cases - No results, many errors, long paths

Error Handling

module.exports = function(results, context) {
  try {
    // Format logic
    return formatResults(results);
  } catch (error) {
    // Fallback to simple format
    console.error("Formatter error:", error);
    return JSON.stringify(results, null, 2);
  }
};

Real-World Examples

Learn from ESLint’s built-in formatters:

stylish

Default formatter with colors and summary

compact

One line per message for easy parsing

json

JSON output for tool integration

html

HTML report generator

junit

JUnit XML for CI systems

checkstyle

Checkstyle XML format

Community Formatters

Explore formatters on npm:

eslint-formatter-table - Table format
eslint-formatter-gitlab - GitLab code quality format
eslint-formatter-pretty - Enhanced stylish formatter
eslint-formatter-summary - Aggregated summary

Next Steps

Create a Plugin

Bundle formatter with rules and configs

Built-in Formatters

Explore all built-in formatters

​Custom Formatters

​When to Create a Formatter

​Formatter Structure

​Synchronous Formatter

​Async Formatter

​The Results Parameter

​LintResult Properties

​LintMessage Properties

​The Context Parameter

​Example: Summary Formatter

​Example: Detailed Formatter

​Example: JSON Formatter

​Example: HTML Report

​Example: JUnit XML

​Using Environment Variables

​Terminal-Friendly Output

​Color Support

​Creating Your Formatter

​Using a Custom Formatter

​Local File

​npm Package

​Packaging a Formatter

​Testing Formatters

​Advanced Patterns

​Piping to External Tools

​Async Formatters

​Stateful Formatters

​Best Practices

​Error Handling

​Real-World Examples

stylish

compact

json

html

junit

checkstyle

​Community Formatters

​Next Steps

Create a Plugin

Built-in Formatters

Custom Formatters

When to Create a Formatter

Formatter Structure

Synchronous Formatter

Async Formatter

The Results Parameter

LintResult Properties

LintMessage Properties

The Context Parameter

Example: Summary Formatter

Example: Detailed Formatter

Example: JSON Formatter

Example: HTML Report

Example: JUnit XML

Using Environment Variables

Terminal-Friendly Output

Color Support

Creating Your Formatter

Using a Custom Formatter

Local File

npm Package

Packaging a Formatter

Testing Formatters

Advanced Patterns

Piping to External Tools

Async Formatters

Stateful Formatters

Best Practices

Error Handling

Real-World Examples

Community Formatters

Next Steps