@equinor/fusion-log

Structured, level-filtered logging utilities for Fusion Framework applications.

@equinor/fusion-log gives you a thin, RxJS-backed logging abstraction with a ready-made console implementation and helpers for resolving log levels from environment variables. Use it whenever you need consistent, configurable log output across modules.

Key Exports

Quick Start

import { ConsoleLogger, LogLevel } from '@equinor/fusion-log';

const logger = new ConsoleLogger('MyApp');
logger.level = LogLevel.Debug;

logger.debug('bootstrapping modules…');
logger.info('application ready');
logger.warn('deprecated API called');
logger.error('failed to fetch config', err);

Log Levels

Higher numeric values are more verbose. Setting logger.level to a given level enables that level and all levels below it.

Configure via Environment Variable

Set FUSION_LOG_LEVEL before your bundler compiles the code so the default level is inlined at build time.

# .env
FUSION_LOG_LEVEL=4        # numeric
# or
FUSION_LOG_LEVEL=debug    # case-insensitive name

[!WARNING] Ensure that FUSION_LOG_LEVEL is set in your package manager's environment variables before compiling the code for production.

Resolution rules (applied once at module load):

1. If FUSION_LOG_LEVEL is set and parsable → use the parsed value.
2. If parsing fails and NODE_ENV === 'development' → fall back to Debug.
3. Otherwise → fall back to Error.
4. If the variable is not set → default to Error.

You can also resolve a level at runtime with resolveLogLevel:

import { resolveLogLevel } from '@equinor/fusion-log';

const level = resolveLogLevel(process.env.MY_LOG_LEVEL ?? 'error');
logger.level = level;

Sub-Loggers

Create child loggers to add component-level context. The child's title is prefixed with the parent's title, and the log level is inherited unless overridden.

const router = logger.createSubLogger('Router');
router.debug('navigated to /dashboard');
// console output: "MyApp::Router  navigated to /dashboard"

const auth = logger.createSubLogger('Auth', undefined, LogLevel.Error);
auth.info('token refreshed'); // suppressed — level is Error

Custom Loggers

Extending Logger

Subclass Logger to route log entries to any destination while reusing level filtering and the RxJS observable pipeline.

import { Logger, LogLevel, type ILogger } from '@equinor/fusion-log';

class RemoteLogger extends Logger {
  constructor(private readonly endpoint: string) {
    super();
    // subscribe to the filtered stream
    this.log.subscribe(({ lvl, msg }) => {
      fetch(this.endpoint, {
        method: 'POST',
        body: JSON.stringify({ level: lvl, messages: msg }),
      });
    });
  }

  protected _createMessage(_lvl: LogLevel, ...msg: unknown[]): unknown[] {
    return msg;
  }

  createSubLogger(title: string): ILogger {
    const sub = new RemoteLogger(this.endpoint);
    sub.level = this.level;
    return sub;
  }
}

Implementing ILogger

If you don't need the RxJS pipeline, implement ILogger directly:

import { type ILogger, LogLevel, defaultLogLevel } from '@equinor/fusion-log';

class NoopLogger implements ILogger {
  level: LogLevel = defaultLogLevel;
  debug() {}
  info() {}
  warn() {}
  error() {}
  createSubLogger(): ILogger {
    return new NoopLogger();
  }
}