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What Are Custom Extractors?

Custom extractors are specialized plugins that enable you to handle file formats that aren’t natively supported by Flatfile’s existing plugins. They process uploaded files, extract structured data, and provide that data for mapping into Sheets as Records. This guide covers everything you need to know to build custom extractors. Common use cases include:
  • Legacy system data exports (custom delimited files, fixed-width formats)
  • Industry-specific formats (healthcare, finance, manufacturing)
  • Multi-format processors (handling various formats in one extractor)
  • Binary file handlers (images with metadata, proprietary formats)

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Architecture Overview

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Core Components

Custom extractors are built using the @flatfile/util-extractor utility, which provides a standardized framework for file processing: 
import { Extractor } from "@flatfile/util-extractor";

export const MyCustomExtractor = (options = {}) => {
  return Extractor(".myformat", "custom", myCustomParser, options);
};
Once you’ve created your extractor, you must register it in a listener to be used. This will ensure that the extractor responds to the file:created event and processes your files. 
// . . . other imports
import { MyCustomExtractor } from "./my-custom-extractor";

export default function (listener) {
  // . . . other listener setup
  listener.use(MyCustomExtractor());
}

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Handling Multiple File Extensions

To support multiple file extensions, use a RegExp pattern: 
// Support both .pipe and .custom extensions
export const MultiExtensionExtractor = (options = {}) => {
  return Extractor(/\.(pipe|custom)$/i, "pipe", parseCustomFormat, options);
};

// Support JSON variants
export const JSONExtractor = (options = {}) => {
  return Extractor(/\.(json|jsonl|jsonlines)$/i, "json", parseJSONFormat, options);
};

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Key Architecture Elements

ComponentPurposeRequired
File ExtensionString or RegExp of supported file extension(s)✓
Extractor TypeString identifier for the extractor type✓
Parser FunctionCore logic that converts file buffer to structured data✓
OptionsConfiguration for chunking, parallelization, and customization-

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Data Flow

  1. File Upload → Flatfile receives file with matching extension
  2. Event Trigger → file:created event fires
  3. Parser Execution → Your parser function processes the file buffer
  4. Data Structuring → Raw data is converted to WorkbookCapture format and provided to Flatfile for mapping into Sheets as Records
  5. Job Completion → Processing status is reported to user

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Getting Started

Remember that custom extractors are powerful tools for handling unique data formats. Start with simple implementations and gradually add complexity as needed. For guidance on building and deploying listeners, see our listener setup guide. For authentication and deployment guidance, see our authentication guide.

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Prerequisites

Install the required packages. You’ll also want to review our listener setup guide if you haven’t created a listener yet. 
npm install @flatfile/util-extractor @flatfile/listener @flatfile/api

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Basic Implementation

Let’s create a simple custom extractor for a pipe-delimited format. This will be used to process files with the .pipe or .psv extension that look like this: 
name|email|phone
John Doe|john@example.com|123-456-7890
Jane Smith|jane@example.com|098-765-4321

import { Extractor } from "@flatfile/util-extractor";

// Parser function - converts Buffer to WorkbookCapture
function parseCustomFormat(buffer) {
  const content = buffer.toString('utf-8');
  const lines = content.split('\n').filter(line => line.trim());
  
  if (lines.length === 0) {
    throw new Error('Empty file');
  }
  
  // First line contains headers
  const headers = lines[0].split('|').map(h => h.trim());
  
  // Remaining lines contain data
  const data = lines.slice(1).map(line => {
    const values = line.split('|').map(v => v.trim());
    const record = {};
    
    headers.forEach((header, index) => {
      record[header] = {
        value: values[index] || ''
      };
    });
    
    return record;
  });
  
  return {
    Sheet1: {
      headers,
      data
    }
  };
}

// Create the extractor
export const CustomPipeExtractor = (options = {}) => {
  return Extractor(/\.(pipe|psv)$/i, "pipe", parseCustomFormat, options);
};
And now let’s import and register it in your listener. 
// . . . other imports
import { CustomPipeExtractor } from "./custom-pipe-extractor";

export default function (listener) {
  // . . . other listener setup
  listener.use(CustomPipeExtractor());
}
That’s it! Your extractor is now registered and will be used to process pipe-delimited files with the .pipe or .psv extension.

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Advanced Examples

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Multi-Sheet Parser

Let’s construct an Extractor to handle files that contain multiple data sections. This will be used to process files with the .multi or .sections extension that look like this: 
---SECTION---
SHEET:Sheet1
name,email,phone
John Doe,john@example.com,123-456-7890
Jane Smith,jane@example.com,098-765-4321
---SECTION---
SHEET:Sheet2
name,email,phone
Jane Doe,jane@example.com,123-456-7891
John Smith,john@example.com,098-765-4322
---SECTION---

function parseMultiSheetFormat(buffer) {
  const content = buffer.toString('utf-8');
  const sections = content.split('---SECTION---');
  
  const workbook = {};
  
  sections.forEach((section, index) => {
    if (!section.trim()) return;
    
    const lines = section.trim().split('\n');
    const sheetName = lines[0].replace('SHEET:', '').trim() || `Sheet${index + 1}`;
    
    const headers = lines[1].split(',').map(h => h.trim());
    const data = lines.slice(2).map(line => {
      const values = line.split(',').map(v => v.trim());
      const record = {};
      
      headers.forEach((header, idx) => {
        record[header] = {
          value: values[idx] || ''
        };
      });
      
      return record;
    });
    
    workbook[sheetName] = { headers, data };
  });
  
  return workbook;
}

export const MultiSheetExtractor = (options = {}) => {
  return Extractor(/\.(multi|sections)$/i, "multi-sheet", parseMultiSheetFormat, options);
};
Now let’s register it in your listener. 
// . . . other imports
import { MultiSheetExtractor } from "./multi-sheet-extractor";

export default function (listener) {
  // . . . other listener setup
  listener.use(MultiSheetExtractor());
}

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Binary Format Handler

This example will be used to process binary files with structured data. This will be used to process binary files with the .bin or .dat extension. Due to the nature of binary format, we can’t easily present a sample import here. 
function parseBinaryFormat(buffer) {
  // Example: Custom binary format with header + records
  let offset = 0;
  
  // Read header (first 16 bytes)
  const magic = buffer.readUInt32LE(offset); offset += 4;
  const version = buffer.readUInt16LE(offset); offset += 2;
  const recordCount = buffer.readUInt32LE(offset); offset += 4;
  const fieldCount = buffer.readUInt16LE(offset); offset += 2;
  
  if (magic !== 0xDEADBEEF) {
    throw new Error('Invalid file format');
  }
  
  // Read field definitions
  const headers = [];
  for (let i = 0; i < fieldCount; i++) {
    const nameLength = buffer.readUInt16LE(offset); offset += 2;
    const name = buffer.toString('utf-8', offset, offset + nameLength);
    offset += nameLength;
    const type = buffer.readUInt8(offset); offset += 1;
    
    headers.push(name);
  }
  
  // Read records
  const data = [];
  for (let i = 0; i < recordCount; i++) {
    const record = {};
    
    headers.forEach(header => {
      const valueLength = buffer.readUInt16LE(offset); offset += 2;
      const value = buffer.toString('utf-8', offset, offset + valueLength);
      offset += valueLength;
      
      record[header] = { value };
    });
    
    data.push(record);
  }
  
  return {
    Sheet1: { headers, data }
  };
}

export const BinaryExtractor = (options = {}) => {
  return Extractor(/\.(bin|dat)$/i, "binary", parseBinaryFormat, options);
};
And, once again, let’s register it in your listener. 
// . . . other imports
import { BinaryExtractor } from "./binary-extractor";

export default function (listener) {
  // . . . other listener setup
  listener.use(BinaryExtractor());
}

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Configuration-Driven Extractor

Create a flexible extractor that can be configured for different formats. This will be used to process files in a manner that handles different delimiters, line endings, and other formatting options. 
function createConfigurableParser(config) {
  return function parseConfigurableFormat(buffer) {
    const content = buffer.toString(config.encoding || 'utf-8');
    let lines = content.split(config.lineDelimiter || '\n');
    
    // Skip header lines if specified
    if (config.skipLines) {
      lines = lines.slice(config.skipLines);
    }
    
    // Filter empty lines
    if (config.skipEmptyLines) {
      lines = lines.filter(line => line.trim());
    }
    
    if (lines.length === 0) {
      throw new Error('No data found');
    }
    
    // Extract headers
    let headers;
    let dataStartIndex = 0;
    
    if (config.explicitHeaders) {
      headers = config.explicitHeaders;
    } else {
      headers = lines[0].split(config.fieldDelimiter || ',').map(h => h.trim());
      dataStartIndex = 1;
    }
    
    // Process data
    const data = lines.slice(dataStartIndex).map(line => {
      const values = line.split(config.fieldDelimiter || ',');
      const record = {};
      
      headers.forEach((header, index) => {
        let value = values[index] || '';
        
        // Apply transformations
        if (config.transforms && config.transforms[header]) {
          value = config.transforms[header](value);
        }
        
        // Type conversion
        if (config.typeConversion) {
          if (!isNaN(value) && value !== '') {
            value = Number(value);
          } else if (value.toLowerCase() === 'true' || value.toLowerCase() === 'false') {
            value = value.toLowerCase() === 'true';
          }
        }
        
        record[header] = { value };
      });
      
      return record;
    });
    
    return {
      [config.sheetName || 'Sheet1']: { headers, data }
    };
  };
}

export const ConfigurableExtractor = (userConfig = {}) => {
  const defaultConfig = {
    encoding: 'utf-8',
    lineDelimiter: '\n',
    fieldDelimiter: ',',
    skipLines: 0,
    skipEmptyLines: true,
    typeConversion: false,
    sheetName: 'Sheet1'
  };
  
  const config = { ...defaultConfig, ...userConfig };
  
  return Extractor(
    config.fileExtension || ".txt", 
    "configurable", 
    createConfigurableParser(config),
    {
      chunkSize: config.chunkSize || 10000,
      parallel: config.parallel || 1
    }
  );
};
Now let’s register two different configurable extractors in our listener. The first will be used to process files with the .custom extension that look like this, while transforming dates and amount values: 
Extraneous text
More extraneous text
name & date & amount
John Doe & 1/1/2021 & 100.00
Jane Smith & 1/2/2021 & 200.00
The second will be used to process files with the .pipe or .special extension that look like this: 
Extraneous text
More extraneous text
name|date|amount
John Doe|2021-01-01|100.00
Jane Smith|2021-01-02|200.00

// . . . other imports
import { ConfigurableExtractor } from "./configurable-extractor";

export default function (listener) {
  // . . . other listener setup

  // Custom extractor with configuration for .custom files
  listener.use(ConfigurableExtractor({
    fileExtension: ".custom",
    fieldDelimiter: " & ",
    skipLines: 2,
    typeConversion: true,
    transforms: {
      'date': (value) => new Date(value).toISOString(),
      'amount': (value) => parseFloat(value).toFixed(2)
    }
  }));

  // Custom extractor with configuration for .pipe and .special files
  listener.use(ConfigurableExtractor({
    fileExtension: /\.(pipe|special)$/i,
    fieldDelimiter: "|",
    skipLines: 2,
    typeConversion: true
  }));
}

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Reference

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API

function Extractor(
  fileExt: string | RegExp,
  extractorType: string,
  parseBuffer: (
    buffer: Buffer,
    options: any
  ) => WorkbookCapture | Promise<WorkbookCapture>,
  options?: Record<string, any>
): (listener: FlatfileListener) => void
ParameterTypeDescription
fileExtstring or RegExpFile extension to process (e.g., ".custom" or /\.(custom|special)$/i)
extractorTypestringIdentifier for the extractor type (e.g., “custom”, “binary”)
parseBufferParserFunctionFunction that converts Buffer to WorkbookCapture
optionsRecord<string, any>Optional configuration object

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Options

OptionTypeDefaultDescription
chunkSizenumber5000Records to process per batch
parallelnumber1Number of concurrent processing chunks
debugbooleanfalseEnable debug logging

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Parser Function Options

Your parseBuffer function receives additional options beyond what you pass to Extractor:
OptionTypeDescription
fileIdstringThe ID of the file being processed
fileExtstringThe file extension (e.g., “.csv”)
headerSelectionEnabledbooleanWhether header selection is enabled for the space

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Data Structures

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WorkbookCapture Structure

The parser function must return a WorkbookCapture object: 
const workbookCapture = {
  "SheetName1": {
    headers: ["field1", "field2", "field3"],
    data: [
      {
        field1: { value: "value1" },
        field2: { value: "value2" },
        field3: { value: "value3" }
      },
      // ... more records
    ]
  },
  "SheetName2": {
    headers: ["col1", "col2"],
    data: [
      {
        col1: { value: "data1" },
        col2: { value: "data2" }
      }
    ]
  }
};

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Cell Value Objects

Each cell value should use the Flatfile.RecordData format: 
const recordData = {
  field1: { value: "john@example.com" },
  field2: { value: "John Doe" },
  field3: { 
    value: "invalid-email",
    messages: [
      {
        type: "error",
        message: "Invalid email format"
      }
    ]
  }
};

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Message Types

TypeDescriptionUI Effect
errorValidation errorRed highlighting, blocks submission
warningWarning messageYellow highlighting, allows submission
infoInformational messageBlue highlighting

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TypeScript Interfaces

type ParserFunction = (
  buffer: Buffer,
  options: any
) => WorkbookCapture | Promise<WorkbookCapture>;

type WorkbookCapture = Record<string, SheetCapture>;

type SheetCapture = {
  headers: string[];
  descriptions?: Record<string, string | null> | null;
  data: Flatfile.RecordData[];
  metadata?: { rowHeaders: number[] };
};

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Troubleshooting Common Issues

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Files Not Processing

Symptoms: Files upload but no extraction occurs Solutions:
  • Verify file extension matches fileExt configuration
  • Check listener is properly deployed and running
  • Enable debug logging to see processing details
const extractor = CustomExtractor({
  debug: true
}); // Make sure file extensions match in the Extractor call

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Parser Errors

Symptoms: Jobs fail with parsing errors Solutions:
  • Add try-catch blocks in parser function
  • Validate input data before processing
  • Return helpful error messages
function parseCustomFormat(buffer) {
  try {
    const content = buffer.toString('utf-8');
    
    if (!content || content.trim() === '') {
      throw new Error('File is empty');
    }
    
    // ... parsing logic
    
  } catch (error) {
    throw new Error(`Parse error: ${error.message}`);
  }
}

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Memory Issues

Symptoms: Large files cause timeouts or memory errors Solutions:
  • Reduce chunk size for large files
  • Implement streaming for very large files
  • Use parallel processing carefully
const extractor = CustomExtractor({
  chunkSize: 1000,  // Smaller chunks
  parallel: 1       // Reduce parallelization
});

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Performance Problems

Symptoms: Slow processing, timeouts Solutions:
  • Optimize parser algorithm
  • Use appropriate chunk sizes
  • Consider parallel processing for I/O-bound operations
// Optimize for large files
const extractor = CustomExtractor({
  chunkSize: 5000,
  parallel: 3
});