Custom Processors

Processors enable ESLint to extract and lint JavaScript from non-JavaScript files. They’re perfect for Markdown code blocks, HTML script tags, or framework-specific file formats.

When to Use Processors

Create a processor when you need to:

Lint JavaScript in Markdown code blocks
Extract scripts from HTML files
Process framework-specific files (Vue, Svelte)
Handle template languages
Transform files before linting

Popular Processors:

@eslint/markdown - Lint JavaScript in Markdown
eslint-plugin-vue - Vue single-file components
eslint-plugin-html - HTML script tags

Processor Structure

A processor is an object with preprocess() and postprocess() methods:

const plugin = {
  meta: {
    name: "eslint-plugin-example",
    version: "1.2.3"
  },
  processors: {
    "processor-name": {
      meta: {
        name: "eslint-processor-name",
        version: "1.2.3"
      },
      
      // Extract JavaScript code
      preprocess(text, filename) {
        return [
          { text: code1, filename: "0.js" },
          { text: code2, filename: "1.js" }
        ];
      },
      
      // Adjust and aggregate messages
      postprocess(messages, filename) {
        return [].concat(...messages);
      },
      
      supportsAutofix: true
    }
  }
};

export default plugin;

Processor Methods

preprocess(text, filename)

function

required

Extracts JavaScript code blocks from the fileParameters:

text (string): File contents
filename (string): File path

Returns: Array of code blocks, each with:

text (string): JavaScript code to lint
filename (string): Virtual filename (should include extension)

preprocess(text, filename) {
  const blocks = extractCodeBlocks(text);
  return blocks.map((code, index) => ({
    text: code,
    filename: `${index}.js`
  }));
}

postprocess(messages, filename)

function

required

Adjusts lint message locations and aggregates resultsParameters:

messages (Message[][]): Two-dimensional array of messages
filename (string): File path

Returns: Flat array of adjusted messages

postprocess(messages, filename) {
  // messages[0] = messages for first code block
  // messages[1] = messages for second code block
  
  return messages
    .flat()
    .map(adjustMessageLocation);
}

supportsAutofix

boolean

Set to true to enable automatic fixes

When supportsAutofix is true, postprocess() must also transform the fix property of messages to reference the original file locations.

Message Structure

Lint messages have this structure:

type LintMessage = {
  line?: number;          // 1-based line number
  column?: number;        // 1-based column number
  endLine?: number;       // 1-based end line
  endColumn?: number;     // 1-based end column
  
  ruleId: string | null;  // Rule ID
  message: string;        // Error message
  severity: 0 | 1 | 2;   // off, warn, error
  
  fix?: {
    range: [number, number];
    text: string;
  };
  
  suggestions?: Array<{
    desc?: string;
    messageId?: string;
    fix: {
      range: [number, number];
      text: string;
    };
  }>;
};

Example: Markdown Processor

Extract JavaScript from Markdown code blocks:

const plugin = {
  meta: {
    name: "eslint-plugin-markdown",
    version: "1.0.0"
  },
  
  processors: {
    markdown: {
      meta: {
        name: "markdown-processor",
        version: "1.0.0"
      },
      
      preprocess(text, filename) {
        const codeBlocks = [];
        const regex = /```(?:javascript|js)\n([\s\S]*?)```/g;
        
        let match;
        while ((match = regex.exec(text)) !== null) {
          const code = match[1];
          const offset = match.index + match[0].indexOf(code);
          
          codeBlocks.push({
            text: code,
            filename: `${codeBlocks.length}.js`,
            offset // Store for postprocess
          });
        }
        
        // Store offsets for postprocessing
        this.offsets = codeBlocks.map(block => block.offset);
        
        return codeBlocks;
      },
      
      postprocess(messages, filename) {
        // Flatten messages from all code blocks
        return messages.flatMap((blockMessages, index) => {
          const offset = this.offsets[index];
          const offsetLines = text.slice(0, offset)
            .split('\n').length - 1;
          
          return blockMessages.map(message => ({
            ...message,
            // Adjust line numbers to original file
            line: message.line + offsetLines,
            endLine: message.endLine
              ? message.endLine + offsetLines
              : undefined
          }));
        });
      },
      
      supportsAutofix: false
    }
  }
};

export default plugin;

Example: HTML Processor

Extract JavaScript from HTML script tags:

const plugin = {
  processors: {
    ".html": {
      preprocess(text, filename) {
        const scripts = [];
        const scriptRegex = /<script[^>]*>([\s\S]*?)<\/script>/gi;
        
        let match;
        while ((match = scriptRegex.exec(text)) !== null) {
          const code = match[1];
          const offset = match.index + match[0].indexOf(code);
          
          // Calculate line offset
          const lineOffset = text.slice(0, offset)
            .split('\n').length - 1;
          
          scripts.push({
            text: code,
            filename: `${scripts.length}.js`,
            lineOffset
          });
        }
        
        this.lineOffsets = scripts.map(s => s.lineOffset);
        return scripts;
      },
      
      postprocess(messages, filename) {
        return messages.flatMap((scriptMessages, index) => {
          const lineOffset = this.lineOffsets[index];
          
          return scriptMessages.map(msg => ({
            ...msg,
            line: (msg.line || 0) + lineOffset,
            endLine: msg.endLine
              ? msg.endLine + lineOffset
              : undefined,
            column: msg.column,
            endColumn: msg.endColumn
          }));
        });
      }
    }
  }
};

Supporting Autofixes

To support automatic fixes, transform fix ranges to original file positions:

processors: {
  "processor-name": {
    preprocess(text, filename) {
      const blocks = extractBlocks(text);
      
      // Store mapping info
      this.blocks = blocks.map(block => ({
        originalOffset: block.offset,
        text: block.text
      }));
      
      return blocks.map((block, i) => ({
        text: block.text,
        filename: `${i}.js`
      }));
    },
    
    postprocess(messages, filename) {
      return messages.flatMap((blockMessages, blockIndex) => {
        const block = this.blocks[blockIndex];
        const originalOffset = block.originalOffset;
        
        return blockMessages.map(message => {
          const adjustedMessage = {
            ...message,
            line: calculateOriginalLine(message.line, originalOffset),
            column: message.column
          };
          
          // Adjust fix ranges
          if (message.fix) {
            adjustedMessage.fix = {
              range: [
                message.fix.range[0] + originalOffset,
                message.fix.range[1] + originalOffset
              ],
              text: message.fix.text
            };
          }
          
          // Adjust suggestion fix ranges
          if (message.suggestions) {
            adjustedMessage.suggestions = message.suggestions.map(sugg => ({
              ...sugg,
              fix: {
                range: [
                  sugg.fix.range[0] + originalOffset,
                  sugg.fix.range[1] + originalOffset
                ],
                text: sugg.fix.text
              }
            }));
          }
          
          return adjustedMessage;
        });
      });
    },
    
    supportsAutofix: true
  }
}

Metadata Objects

Plugin Meta

Define at plugin level:

const plugin = {
  meta: {
    name: "eslint-plugin-example",
    version: "1.2.3",
    namespace: "example"
  },
  processors: { /* ... */ }
};

Processor Meta

Define for each processor:

processors: {
  "processor-name": {
    meta: {
      name: "eslint-processor-name",
      version: "1.2.3"
    },
    preprocess() { /* ... */ },
    postprocess() { /* ... */ }
  }
}

Why Both Meta Objects?The plugin meta is used when the processor is referenced by string (e.g., "example/processor-name"). The processor meta is used when the processor object is passed directly in configuration.

Creating Your Processor

Identify Code Blocks

Determine how to extract JavaScript from your file format:

function extractCodeBlocks(text) {
  // For Markdown
  const blocks = [];
  const regex = /```js\n([\s\S]*?)```/g;
  
  let match;
  while ((match = regex.exec(text))) {
    blocks.push({
      code: match[1],
      offset: match.index,
      line: text.slice(0, match.index)
        .split('\n').length
    });
  }
  
  return blocks;
}

Implement Preprocess

Extract and return code blocks:

preprocess(text, filename) {
  const blocks = extractCodeBlocks(text);
  
  // Store metadata for postprocess
  this.blockMetadata = blocks.map(b => ({
    offset: b.offset,
    line: b.line
  }));
  
  return blocks.map((block, i) => ({
    text: block.code,
    filename: `${i}.js`
  }));
}

Implement Postprocess

Adjust message locations:

postprocess(messages, filename) {
  return messages.flatMap((blockMsgs, i) => {
    const metadata = this.blockMetadata[i];
    
    return blockMsgs.map(msg => ({
      ...msg,
      line: msg.line + metadata.line - 1,
      endLine: msg.endLine
        ? msg.endLine + metadata.line - 1
        : undefined
    }));
  });
}

Test Thoroughly

Test with various code block configurations:

const processor = plugin.processors.markdown;

const input = `
# Title

\`\`\`javascript
const x = 1;
\`\`\`

Text

\`\`\`js
const y = 2;
\`\`\`
`;

const blocks = processor.preprocess(input, "test.md");
console.log(blocks);
// [
//   { text: "const x = 1;\n", filename: "0.js" },
//   { text: "const y = 2;\n", filename: "1.js" }
// ]

Using Processors in Configuration

Configure ESLint to use your processor:

eslint.config.js

import example from "eslint-plugin-example";

export default [
  {
    files: ["**/*.md"],
    plugins: {
      example
    },
    processor: "example/markdown"
  },
  {
    // Rules for extracted JavaScript
    files: ["**/*.md/*.js"],
    rules: {
      "no-console": "off",
      "no-unused-vars": "warn"
    }
  }
];

Virtual FilenamesESLint uses the filename from preprocessed blocks to match configuration. Use patterns like **/*.md/*.js to target extracted code.

Advanced Patterns

Multiple File Types

Handle different file types with one plugin:

const plugin = {
  processors: {
    ".md": markdownProcessor,
    ".html": htmlProcessor,
    ".vue": vueProcessor
  }
};

Conditional Processing

Process only certain code blocks:

preprocess(text, filename) {
  const blocks = [];
  const regex = /```(\w+)\n([\s\S]*?)```/g;
  
  let match;
  while ((match = regex.exec(text))) {
    const lang = match[1];
    const code = match[2];
    
    // Only process JavaScript and TypeScript
    if (lang === 'javascript' || lang === 'js' || lang === 'typescript' || lang === 'ts') {
      blocks.push({
        text: code,
        filename: `${blocks.length}.${lang === 'typescript' || lang === 'ts' ? 'ts' : 'js'}`
      });
    }
  }
  
  return blocks;
}

Preserving Indentation

Maintain original indentation in extracted code:

preprocess(text, filename) {
  const blocks = extractBlocks(text);
  
  return blocks.map((block, i) => {
    // Calculate base indentation
    const lines = block.text.split('\n');
    const firstLine = lines.find(line => line.trim());
    const baseIndent = firstLine ? firstLine.match(/^\s*/)[0].length : 0;
    
    // Remove base indentation
    const dedented = lines
      .map(line => line.slice(baseIndent))
      .join('\n');
    
    return {
      text: dedented,
      filename: `${i}.js`
    };
  });
}

Testing Processors

Unit Tests

const assert = require("assert");
const plugin = require("./index");

const processor = plugin.processors.markdown;

describe("Markdown Processor", () => {
  describe("preprocess", () => {
    it("should extract code blocks", () => {
      const input = `
        # Title
        \`\`\`javascript
        const x = 1;
        \`\`\`
      `;
      
      const blocks = processor.preprocess(input, "test.md");
      
      assert.strictEqual(blocks.length, 1);
      assert.strictEqual(blocks[0].text.trim(), "const x = 1;");
    });
    
    it("should handle multiple blocks", () => {
      const input = `
        \`\`\`js
        const a = 1;
        \`\`\`
        
        \`\`\`js
        const b = 2;
        \`\`\`
      `;
      
      const blocks = processor.preprocess(input, "test.md");
      assert.strictEqual(blocks.length, 2);
    });
  });
  
  describe("postprocess", () => {
    it("should adjust line numbers", () => {
      const messages = [
        [{
          line: 1,
          column: 7,
          message: "Error",
          ruleId: "no-unused-vars",
          severity: 2
        }]
      ];
      
      // Simulate processor state
      processor.offsets = [50]; // Code block starts at char 50
      
      const result = processor.postprocess(messages, "test.md");
      
      assert(result[0].line > 1); // Line adjusted
    });
  });
});

Integration Tests

const { ESLint } = require("eslint");
const plugin = require("./index");

const eslint = new ESLint({
  overrideConfigFile: true,
  overrideConfig: [
    {
      files: ["**/*.md"],
      plugins: { example: plugin },
      processor: "example/markdown"
    },
    {
      files: ["**/*.md/*.js"],
      rules: {
        "no-unused-vars": "error"
      }
    }
  ]
});

const results = await eslint.lintText(
  `
  # Test
  
  \`\`\`javascript
  const unused = 1;
  \`\`\`
  `,
  { filePath: "test.md" }
);

console.log(results[0].messages);

Performance Tips

Processors run on every file. Optimize performance:

Cache regex patterns - Compile once, use many times
Minimize string operations - Avoid unnecessary slicing/splitting
Use efficient parsing - Consider parser libraries for complex formats
Limit regex backtracking - Use possessive quantifiers

Optimized Extraction

// Good: Compile regex once
const CODE_BLOCK_REGEX = /```(?:javascript|js)\n([\s\S]*?)```/g;

preprocess(text, filename) {
  const blocks = [];
  let match;
  
  // Reuse compiled regex
  CODE_BLOCK_REGEX.lastIndex = 0;
  while ((match = CODE_BLOCK_REGEX.exec(text))) {
    blocks.push({
      text: match[1],
      filename: `${blocks.length}.js`
    });
  }
  
  return blocks;
}

Common Pitfalls

Incorrect Line Number Adjustments

Always calculate line offsets based on the original file:

// Calculate line offset
const lineOffset = text.slice(0, blockOffset)
  .split('\n').length - 1;

// Adjust message
message.line += lineOffset;

Forgetting to Flatten Messages

postprocess() receives a 2D array but must return a 1D array:

// Wrong
return messages;

// Right
return messages.flat();
// or
return [].concat(...messages);

Not Setting supportsAutofix

If you want fixes to work, set supportsAutofix: true and adjust fix ranges in postprocess().

Losing Message Properties

Preserve all message properties when adjusting:

return blockMessages.map(msg => ({
  ...msg, // Preserve all properties
  line: adjustedLine,
  endLine: adjustedEndLine
}));

Real-World Examples

@eslint/markdown

Official Markdown processor

eslint-plugin-vue

Vue single-file component processor

eslint-plugin-html

HTML script tag processor

eslint-plugin-svelte

Svelte component processor

Next Steps

Create a Plugin

Package your processor for distribution

Configure Processors

Learn how users configure processors

Get Started

Using ESLint

Extending ESLint

Guides

Custom Processors

Custom Processors

When to Use Processors

Processor Structure

Processor Methods

Message Structure

Example: Markdown Processor

Example: HTML Processor

Supporting Autofixes

Metadata Objects

Plugin Meta

Processor Meta

Creating Your Processor

Using Processors in Configuration

Advanced Patterns

Multiple File Types

Conditional Processing

Preserving Indentation

Testing Processors

Unit Tests

Integration Tests

Performance Tips

Optimized Extraction

Common Pitfalls

Real-World Examples

@eslint/markdown

eslint-plugin-vue

eslint-plugin-html

eslint-plugin-svelte

Next Steps

Create a Plugin

Configure Processors

Get Started

Using ESLint

Extending ESLint

Guides

​Custom Processors

​When to Use Processors

​Processor Structure

​Processor Methods

​Message Structure

​Example: Markdown Processor

​Example: HTML Processor

​Supporting Autofixes

​Metadata Objects

​Plugin Meta

​Processor Meta

​Creating Your Processor

​Using Processors in Configuration

​Advanced Patterns

​Multiple File Types

​Conditional Processing

​Preserving Indentation

​Testing Processors

​Unit Tests

​Integration Tests

​Performance Tips

​Optimized Extraction

​Common Pitfalls

​Real-World Examples

@eslint/markdown

eslint-plugin-vue

eslint-plugin-html

eslint-plugin-svelte

​Next Steps

Create a Plugin

Configure Processors

Custom Processors

When to Use Processors

Processor Structure

Processor Methods

Message Structure

Example: Markdown Processor

Example: HTML Processor

Supporting Autofixes

Metadata Objects

Plugin Meta

Processor Meta

Creating Your Processor

Using Processors in Configuration

Advanced Patterns

Multiple File Types

Conditional Processing

Preserving Indentation

Testing Processors

Unit Tests

Integration Tests

Performance Tips

Optimized Extraction

Common Pitfalls

Real-World Examples

Next Steps