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Version: 1.0.0

FilterHandler

Advanced conditional logging with custom filter functions.

Overview​

FilterHandler allows you to filter log records based on custom logic beyond simple level filtering. Filter by message content, attributes, context, or any custom criteria.

Basic Usage​

import { New, FilterHandler, JSONHandler } from '@omdxp/jslog';

const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => {
    // Only log records with 'critical' in the message
    return record.message.includes('critical');
  }
}));

logger.info('Normal operation');      // Filtered out
logger.info('Critical failure!');     // Logged

Configuration Options​

interface FilterHandlerOptions {
  handler: Handler;
  filter: (record: Record) => boolean;
}

Filter Functions​

Level-Based Filtering​

import { New, FilterHandler, JSONHandler, Level } from '@omdxp/jslog';

const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => {
    // Log ERROR and above, OR anything with 'important'
    return record.level >= Level.ERROR || 
           record.message.includes('important');
  }
}));

Attribute-Based Filtering​

import { New, FilterHandler, JSONHandler, String } from '@omdxp/jslog';

const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => {
    // Only log records from production environment
    const env = record.attrs.find(a => a.key === 'env');
    return env?.value === 'production';
  }
}));

logger.info('Test', String('env', 'development'));  // Filtered
logger.info('Live', String('env', 'production'));   // Logged

Time-Based Filtering​

const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => {
    // Only log during business hours (9 AM - 5 PM)
    const hour = new Date().getHours();
    return hour >= 9 && hour < 17;
  }
}));

Common Patterns​

Exclude Noisy Logs​

const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => {
    // Exclude health check logs
    return !record.message.includes('health check') &&
           !record.message.includes('ping');
  }
}));

User-Based Filtering​

const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => {
    // Only log for admin users
    const userId = record.attrs.find(a => a.key === 'userId');
    return isAdmin(userId?.value);
  }
}));

Sampling with Attributes​

const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => {
    // Always log errors, sample everything else at 10%
    if (record.level >= Level.ERROR) {
      return true;
    }
    return Math.random() < 0.1;
  }
}));

Environment-Specific Filtering​

const isDev = process.env.NODE_ENV === 'development';

const logger = New(new FilterHandler({
  handler: new ColorHandler(),
  filter: (record) => {
    if (isDev) {
      // Log everything in development
      return true;
    }
    // In production, only log WARN and above
    return record.level >= Level.WARN;
  }
}));

Combining Filters​

Multiple Conditions​

const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => {
    // Complex filter logic
    const isError = record.level >= Level.ERROR;
    const isCritical = record.message.includes('critical');
    const hasUserId = record.attrs.some(a => a.key === 'userId');
    
    return isError || (isCritical && hasUserId);
  }
}));

Chainable Filters​

import { New, FilterHandler, JSONHandler } from '@omdxp/jslog';

// First filter: Level
const levelFiltered = new FilterHandler({
  handler: new JSONHandler(),
  filter: (r) => r.level >= Level.INFO
});

// Second filter: Message content
const logger = New(new FilterHandler({
  handler: levelFiltered,
  filter: (r) => !r.message.includes('debug')
}));

Use Cases​

Rate Limiting Per User​

const userCounters = new Map<string, { count: number; resetAt: number }>();

const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => {
    const userId = record.attrs.find(a => a.key === 'userId')?.value as string;
    if (!userId) return true;
    
    const now = Date.now();
    const counter = userCounters.get(userId);
    
    if (!counter || now > counter.resetAt) {
      userCounters.set(userId, { count: 1, resetAt: now + 60000 });
      return true;
    }
    
    if (counter.count < 100) {
      counter.count++;
      return true;
    }
    
    return false; // Rate limit exceeded
  }
}));

Geographic Filtering​

const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => {
    const region = record.attrs.find(a => a.key === 'region')?.value;
    // Only log from specific regions
    return ['us-east-1', 'eu-west-1'].includes(region as string);
  }
}));

Error Code Filtering​

const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => {
    const statusCode = record.attrs.find(a => a.key === 'statusCode')?.value;
    // Only log 4xx and 5xx errors
    return statusCode >= 400;
  }
}));

Performance Considerations​

Keep filter functions fast:

// Good: Simple, fast filter
const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => record.level >= Level.WARN
}));

// Bad: Slow filter with heavy computation
const logger = New(new FilterHandler({
  handler: new JSONHandler(),
  filter: (record) => {
    // Don't do this!
    const result = expensiveComputation(record);
    return result.shouldLog;
  }
}));

Best Practices​

Do:

  • Keep filter functions simple and fast
  • Use early returns for performance
  • Document your filter logic
  • Test filters thoroughly

Don't:

  • Perform expensive operations in filters
  • Mutate the record in filter functions
  • Use filters for transformation (use middleware instead)
  • Forget that filtered logs are lost forever

See Also​