Seq Logging for JavaScript

Common errors

• TypeError: require is not a function

  cause Attempting to use CommonJS `require()` to import `seq-logging` in a project configured for ESM, or in `seq-logging` v3+ which is ESM-only.
  fix Change `const { Logger } = require('seq-logging');` to `import { Logger } from 'seq-logging';` and ensure your project is set up for ES Modules.

• ReferenceError: process is not defined

  cause Attempting to use `process` global in a browser environment without proper polyfill or a Node.js specific build setup.
  fix When building for browsers, avoid direct use of Node.js globals like `process`. If environment variables are needed, pass them in at build time or use a browser-compatible method. Alternatively, ensure your bundler provides a `process` polyfill.

Warnings

• breaking Version 3.0.0 and later are ESM-only, meaning CommonJS `require()` syntax is no longer supported for importing the library. Attempting to use `require()` will result in import errors.
  Fix: Migrate your import statements to use ES module syntax (e.g., `import { Logger } from 'seq-logging'`). If using Node.js, ensure your project is configured for ESM (e.g., `"type": "module"` in package.json or using `.mjs` file extension).
• gotcha Events are buffered internally for asynchronous batching. If the application exits before `close()` or `flush()` is called, buffered events may be lost.
  Fix: Always call `logger.close()` or `logger.flush()` before your application terminates to ensure all pending events are sent to Seq. Both methods return a Promise, which should be awaited.
• gotcha When targeting browser environments, a different entry point must be used (`seq-logging/browser`) compared to Node.js environments (`seq-logging`).
  Fix: Ensure your build system (e.g., Webpack, Rollup) or direct imports use `import { Logger } from 'seq-logging/browser'` when bundling for browsers, and `import { Logger } from 'seq-logging'` for Node.js.
• gotcha The package requires Node.js version 14.18 or higher. Running on older Node.js versions may lead to unexpected errors or compatibility issues.
  Fix: Upgrade your Node.js environment to version 14.18.0 or newer to ensure full compatibility and access to required features.

Install

• npm install seq-logging npm
• yarn add seq-logging yarn
• pnpm add seq-logging pnpm

Imports

• Logger
  wrong:

  const { Logger } = require('seq-logging')

  correct:

  import { Logger } from 'seq-logging'

  Starting with v3, seq-logging is an ESM-only package. CommonJS `require()` is not supported for the main entry point.
• Logger
  wrong:

  const { Logger } = require('seq-logging/browser')

  correct:

  import { Logger } from 'seq-logging/browser'

  For browser environments, use the dedicated `seq-logging/browser` entry point. CommonJS `require()` is not supported for v3.
• LoggerOptions

  import type { LoggerOptions } from 'seq-logging'

  Type-only import for configuring the Logger instance, especially when using TypeScript.

Quickstart

Initializes a Seq Logger and emits a structured log event with properties, demonstrating basic usage and proper shutdown.

import process from 'process';
import { Logger } from 'seq-logging';

const logger = new Logger({
    serverUrl: process.env.SEQ_SERVER_URL ?? 'http://localhost:5341',
    apiKey: process.env.SEQ_API_KEY ?? undefined, // Optional
    requestTimeout: 30000 // default to 30s
});

logger.emit({
    timestamp: new Date(),
    level: 'Information',
    messageTemplate: 'Hello from {appName}! It is {currentTime}.',
    properties: {
        appName: 'MySeqApp',
        currentTime: new Date().toISOString(),
        user: process.env.USERNAME ?? 'anonymous'
    }
});

// Ensure all buffered events are sent before exiting
logger.close().then(() => console.log('Logger closed successfully.'));