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* The possible levels for the Logger, inspired by RFC5424, with an additional 'trace' level for ultra-detailed logging.
*
* The severity levels are assumed to be numerically ascending from most important to least important.
*
* Level descriptions:
*
* ```
* trace - Extremely detailed debugging information (e.g., per-iteration or per-call tracing)
* debug - Detailed debugging information (e.g., function entry/exit points)
* info - General informational messages (e.g., operation progress updates)
* notice - Normal but significant events (e.g., configuration changes)
* warning - Warning conditions (e.g., deprecated feature usage)
* error - Error conditions (e.g., operation failures)
* critical - Critical conditions (e.g., system component failures)
* alert - Action must be taken immediately (e.g., data corruption detected)
* emergency - System is unusable (e.g., complete system failure)
* ```
*/
export type LogLevel = 'trace' | 'debug' | 'info' | 'notice' | 'warning' | 'error' | 'critical' | 'alert' | 'emergency';
/**
* Type representing the content of a log entry. Can be a primitive value or a function that returns a primitive value.
*/
export type LogMessage = string | number | boolean | (() => string | number | boolean);
/**
* Type representing the template strings of a log entry. Can be a TemplateStringsArray or a function that returns a
* TemplateStringsArray.
*/
export type LogTemplateStringsArray = TemplateStringsArray | (() => TemplateStringsArray);
/**
* Generic logger interface that provides methods for logging entries at different severity levels.
*
* The logger supports a filtering mechanism based on severity levels:
*
* - Each logger has a configurable `level` property which sets the minimum severity level for emitted logs
* - Log entries with a severity level below the logger's configured level are filtered out
* - When set to 'off', no log entries are emitted regardless of their level
*
* The severity filtering follows this hierarchy (from lowest to highest): debug < info < notice < warning < error <
* critical < alert < emergency
*
* For example, if the logger's level is set to 'warning':
*
* - Entries logged with debug(), info(), or notice() methods will be filtered out
* - Entries logged with warning(), error(), critical(), alert(), or emergency() will be emitted
*/
export interface Logger {
/**
* The current minimum severity level of the logger.
*
* Log entries with a level below this threshold will not be emitted. For example, setting the level to `info` (which
* is the default level) will cause `debug` logs to be ignored.
*
* If set to 'off', no log entries are emitted regardless of level.
*/
readonly level: LogLevel | 'off';
/**
* Sets additional arguments to be included with the next log entry. This is particularly useful with the template
* string method, because it enables fluent chaining while still passing additional arguments.
*
* @example
*
* ```ts
* // Pass an error object with a template literal
* logger.args(error).e`Failed to connect to database at ${timestamp}`;
*
* // Pass multiple arguments
* logger.args(user, requestId, timestamp).i`User ${user.name} logged in`;
*
* // Pass contextual data
* logger.args({ userId, requestId }).d`Processing request for item ${itemId}`;
* ```
*
* @param args Arguments to be included with the next log entry
* @returns A logger instance for chaining with template literal methods
*/
readonly args: (...args: unknown[]) => Logger;
/**
* Logs a trace-level entry for extremely detailed debugging information (e.g., per-iteration or call tracing).
*
* Trace entries are only emitted if the logger's level is set to `trace`.
*
* The entry content can be either a direct value or a function that returns a value. Using a function is recommended
* when the content is expensive to construct, as the function will only be invoked if the entry will actually be
* emitted based on the current logger level.
*
* @example
*
* ```ts
* logger.trace('Loop iteration start');
* logger.trace(() => `Iteration ${i} state: ${JSON.stringify(state)}`);
* ```
*
* @param message The entry content or function that returns the content
* @param args Additional arguments to include in the log entry
*/
readonly trace: (message: LogMessage, ...args: unknown[]) => void;
/**
* Logs a trace-level entry using a template string, which is only computed if the current log level (`logger.level`)
* is `trace`.
*
* Values in template literals are automatically formatted:
*
* - Error objects will show their message text (e.g., `${new Error('fail')}` becomes 'fail')
* - Objects with a `message` property will display that message
* - Functions are executed and their return values formatted
* - All other values are converted to their string representation
*
* @example
*
* ```ts
* logger.t`Entering loop body`;
* logger.t`Iteration ${i} with state: ${state}`;
* ```
*
* @param strings The template strings
* @param values The values to be interpolated into the template
*/
readonly t: (strings: LogTemplateStringsArray, ...values: unknown[]) => void;
/**
* Logs a debug-level entry for detailed debugging information (e.g., function entry/exit points).
*
* Debug entries are only emitted if the logger's level is set to `debug` or `trace`.
*
* The entry content can be either a direct value or a function that returns a value. Using a function is recommended
* when the content is expensive to construct, as the function will only be invoked if the entry will actually be
* emitted based on the current logger level.
*
* @example
*
* ```ts
* logger.debug('This is a debug entry');
* logger.debug(() => `Debug entry with computed value: ${expensiveOperation()}`);
* ```
*
* @param message The entry content or function that returns the content
* @param args Additional arguments to include in the log entry
*/
readonly debug: (message: LogMessage, ...args: unknown[]) => void;
/**
* Logs a debug-level entry using a template string, which is only computed if the current log level (`logger.level`)
* is `debug` or `trace`.
*
* Values in template literals are automatically formatted:
*
* - Error objects will show their message text (e.g., `${new Error('Connection failed')}` becomes 'Connection failed')
* - Objects with a `message` property will display that message
* - Functions are executed and their return values formatted using the rules above
* - All other values are converted to their string representation
*
* @example
*
* ```ts
* // Basic template literal
* logger.d`Debug entry with simple text`;
*
* // With computed values
* logger.d`Debug entry with computed value: ${expensiveOperation()}`;
*
* // With error objects - message is automatically extracted
* const error = new Error('Something went wrong');
* logger.d`Operation failed: ${error}`; // Logs: "Operation failed: Something went wrong"
* ```
*
* @param strings The template strings
* @param values The values to be interpolated into the template
*/
readonly d: (strings: LogTemplateStringsArray, ...values: unknown[]) => void;
/**
* Logs an info-level entry for general informational messages (e.g., operation progress updates).
*
* Info entries are only emitted if the logger's level is set to `info`, `debug`, or `trace`.
*
* The entry content can be either a direct value or a function that returns a value. Using a function is recommended
* when the message is expensive to construct, as the function will only be invoked if the entry will actually be
* emitted based on the current logger level.
*
* @example
*
* ```ts
* logger.info('Operation started');
* logger.info(() => `Processing item ${itemId} at ${new Date().toISOString()}`);
* ```
*
* @param message The entry content or function that returns the content
* @param args Additional arguments to include in the log entry
*/
readonly info: (message: LogMessage, ...args: unknown[]) => void;
/**
* Logs an info-level entry using a template string, which is only computed if the current log level (`logger.level`)
* is `info`, `debug`, or `trace`.
*
* Values in template literals are automatically formatted:
*
* - Error objects will show their message text (e.g., `${new Error('Connection failed')}` becomes 'Connection failed')
* - Objects with a `message` property will display that message
* - Functions are executed and their return values formatted using the rules above
* - All other values are converted to their string representation
*
* @example
*
* ```ts
* // Basic template literal
* logger.i`Operation started`;
*
* // With computed values
* logger.i`Processing item ${itemId} at ${new Date().toISOString()}`;
*
* // With error objects - message is automatically extracted
* const error = new Error('Missing configuration');
* logger.i`Config status: ${error}`; // Logs: "Config status: Missing configuration"
* ```
*
* @param strings The template strings
* @param values The values to be interpolated into the template
*/
readonly i: (strings: LogTemplateStringsArray, ...values: unknown[]) => void;
/**
* Logs a notice-level entry for normal but significant events (e.g., configuration changes).
*
* Notice entries are only emitted if the logger's level is set to `notice`, `info`, `debug`, or `trace`.
*
* The entry content can be either a direct value or a function that returns a value. Using a function is recommended
* when the message is expensive to construct, as the function will only be invoked if the entry will actually be
* emitted based on the current logger level.
*
* @example
*
* ```ts
* logger.notice('Configuration updated');
* logger.notice(() => `User ${userId} changed settings at ${new Date().toISOString()}`);
* ```
*
* @param message The entry content or function that returns the content
* @param args Additional arguments to include in the log entry
*/
readonly notice: (message: LogMessage, ...args: unknown[]) => void;
/**
* Logs a notice-level entry using a template string, which is only computed if the current log level (`logger.level`)
* is `notice`, `info`, `debug`, or `trace`.
*
* Values in template literals are automatically formatted:
*
* - Error objects will show their message text (e.g., `${new Error('Connection failed')}` becomes 'Connection failed')
* - Objects with a `message` property will display that message
* - Functions are executed and their return values formatted using the rules above
* - All other values are converted to their string representation
*
* @example
*
* ```ts
* // Basic template literal
* logger.n`Configuration updated`;
*
* // With computed values
* logger.n`User ${userId} changed settings at ${new Date().toISOString()}`;
*
* // With error objects - message is automatically extracted
* const warningObj = { message: 'Almost reached quota' };
* logger.n`Usage warning: ${warningObj}`; // Logs: "Usage warning: Almost reached quota"
* ```
*
* @param strings The template strings
* @param values The values to be interpolated into the template
*/
readonly n: (strings: LogTemplateStringsArray, ...values: unknown[]) => void;
/**
* Logs a warning-level entry for warning conditions (e.g., deprecated feature usage).
*
* Warning entries are only emitted if the logger's level is set to `warning`, `notice`, `info`, `debug`, or `trace`.
*
* The entry content can be either a direct value or a function that returns a value. Using a function is recommended
* when the message is expensive to construct, as the function will only be invoked if the entry will actually be
* emitted based on the current logger level.
*
* The warning entry content can be:
*
* - A direct value that is logged as is,
* - A function that returns a value, useful for messages that are expensive to compute,
* - An Error object, which is automatically stringified and added as an additional argument,
* - An object with an `error` property that is logged as a string and added as an additional argument - use this
* whenever logging an error of type unknown.
*
* @example
*
* ```ts
* logger.warning('Feature X is deprecated and will be removed in version 2.0');
* logger.warning(() => `Slow operation detected: ${operation} took ${duration}ms`);
* logger.warning(new Error('Connection timeout... Retrying...'), error);
* logger.warning({ error: ['Connection timeout... Retrying...', errorCode] });
* ```
*
* @param input The entry content or function that returns the content
* @param args Additional arguments to include in the log entry
*/
readonly warning: (input: LogMessage | Error | { error: unknown }, ...args: unknown[]) => void;
/**
* Logs a warning-level entry using a template string, which is only computed if the current log level
* (`logger.level`) is `warning`, `notice`, `info`, `debug`, or `trace`.
*
* Values in template literals are automatically formatted:
*
* - Error objects will show their message text (e.g., `${new Error('Connection failed')}` becomes 'Connection failed')
* - Objects with a `message` property will display that message
* - Functions are executed and their return values formatted using the rules above
* - All other values are converted to their string representation
*
* @example
*
* ```ts
* // Basic template literal
* logger.w`Feature X is deprecated and will be removed in version 2.0`;
*
* // With computed values
* logger.w`Slow operation detected: ${operation} took ${duration}ms`;
*
* // With error objects - message is automatically extracted
* const deprecationError = new Error('API version will be removed soon');
* logger.w`API usage warning: ${deprecationError}`; // Logs: "API usage warning: API version will be removed soon"
* ```
*
* @param strings The template strings
* @param values The values to be interpolated into the template
*/
readonly w: (strings: LogTemplateStringsArray, ...values: unknown[]) => void;
/**
* Logs an error-level entry for error conditions (e.g., operation failures).
*
* Error entries are only emitted if the logger's level is set to `error`, `warning`, `notice`, `info`, `debug`, or
* `trace`.
*
* The error entry content can be:
*
* - A direct value that is logged as is,
* - A function that returns a value, useful for messages that are expensive to compute,
* - An Error object, which is automatically stringified and added as an additional argument,
* - An object with an `error` property that is logged as a string and added as an additional argument - use this
* whenever logging an error of type unknown.
*
* @example
*
* ```ts
* logger.error('Failed to connect to database');
* logger.error(() => `Operation failed at ${new Date().toISOString()}`);
* logger.error(new Error('Connection timeout'), error);
* logger.error({ error: ['Database connection failed', errorCode] });
* ```
*
* @param input The error content, Error object, or function that returns content
* @param args Additional arguments to include in the log entry
*/
readonly error: (input: LogMessage | Error | { error: unknown }, ...args: unknown[]) => void;
/**
* Logs an error-level entry using a template string, which is only computed if the current log level (`logger.level`)
* is `error`, `warning`, `notice`, `info`, `debug`, or `trace`.
*
* Values in template literals are automatically formatted:
*
* - Error objects will show their message text (e.g., `${new Error('Connection failed')}` becomes 'Connection failed')
* - Objects with a `message` property will display that message
* - Functions are executed and their return values formatted using the rules above
* - All other values are converted to their string representation
*
* @example
*
* ```ts
* // Basic template literal
* logger.e`Failed to connect to database`;
*
* // With automatic date formatting
* logger.e`Operation failed at ${new Date()}`;
*
* // With error objects - message is automatically extracted
* const error = new Error('Connection timeout');
* logger.e`Database error: ${error}`; // Logs: "Database error: Connection timeout"
*
* // Combined with args() to include the error object in the log
* logger.args(error).e`An error occurred while performing the operation: ${error}`;
* ```
*
* @param strings The template strings
* @param values The values to be interpolated into the template
*/
readonly e: (strings: LogTemplateStringsArray, ...values: unknown[]) => void;
/**
* Logs a critical-level entry for critical conditions (e.g., system component failures).
*
* Critical entries are only emitted if the logger's level is set to `critical`, `error`, `warning`, `notice`, `info`,
* , `debug`, or `trace`.
*
* The critical entry content can be:
*
* - A direct value that is logged as is,
* - A function that returns a value, useful for messages that are expensive to compute,
* - An Error object, which is automatically stringified and added as an additional argument,
* - An object with an `error` property that is logged as a string and added as an additional argument - use this
* whenever logging an error of type unknown.
*
* @example
*
* ```ts
* logger.critical('Database connection pool exhausted');
* logger.critical(() => `System memory usage critical: ${memoryUsage}%`);
* logger.critical(new Error('Connection timeout'), error);
* logger.critical({ error: ['Database connection failed', errorCode] });
* ```
*
* @param input The entry content or function that returns the content
* @param args Additional arguments to include in the log entry
*/
readonly critical: (input: LogMessage | Error | { error: unknown }, ...args: unknown[]) => void;
/**
* Logs a critical-level entry using a template string, which is only computed if the current log level
* (`logger.level`) is `critical`, `error`, `warning`, `notice`, `info`, `debug`, or `trace`.
*
* Values in template literals are automatically formatted:
*
* - Error objects will show their message text (e.g., `${new Error('Connection failed')}` becomes 'Connection failed')
* - Objects with a `message` property will display that message
* - Functions are executed and their return values formatted using the rules above
* - All other values are converted to their string representation
*
* @example
*
* ```ts
* // Basic template literal
* logger.c`Database connection pool exhausted`;
*
* // With computed values
* logger.c`System memory usage critical: ${memoryUsage}%`;
*
* // With error objects - message is automatically extracted
* const criticalError = new Error('Connection pool saturated');
* logger.c`Resource exhaustion: ${criticalError}`; // Logs: "Resource exhaustion: Connection pool saturated"
* ```
*
* @param strings The template strings
* @param values The values to be interpolated into the template
*/
readonly c: (strings: LogTemplateStringsArray, ...values: unknown[]) => void;
/**
* Logs an alert-level entry for conditions where action must be taken immediately (e.g., data corruption detected).
*
* Alert entries are only emitted if the logger's level is set to `alert`, `critical`, `error`, `warning`, `notice`,
* `info`, `debug`, or `trace`.
*
* The alert entry content can be:
*
* - A direct value that is logged as is,
* - A function that returns a value, useful for messages that are expensive to compute,
* - An Error object, which is automatically stringified and added as an additional argument,
* - An object with an `error` property that is logged as a string and added as an additional argument - use this
* whenever logging an error of type unknown.
*
* @example
*
* ```ts
* logger.alert('Data corruption detected in customer database');
* logger.alert(() => `Security breach detected from IP ${ipAddress}`);
* logger.alert(new Error('Connection timeout'), error);
* logger.alert({ error: ['Database connection failed', errorCode] });
* ```
*
* @param input The entry content or function that returns the content
* @param args Additional arguments to include in the log entry
*/
readonly alert: (input: LogMessage | Error | { error: unknown }, ...args: unknown[]) => void;
/**
* Logs an alert-level entry using a template string, which is only computed if the current log level (`logger.level`)
* is `alert`, `critical`, `error`, `warning`, `notice`, `info`, `debug`, or `trace`.
*
* Values in template literals are automatically formatted:
*
* - Error objects will show their message text (e.g., `${new Error('Connection failed')}` becomes 'Connection failed')
* - Objects with a `message` property will display that message
* - Functions are executed and their return values formatted using the rules above
* - All other values are converted to their string representation
*
* @example
*
* ```ts
* // Basic template literal
* logger.a`Data corruption detected in customer database`;
*
* // With computed values
* logger.a`Security breach detected from IP ${ipAddress}`;
*
* // With error objects - message is automatically extracted
* const securityError = new Error('Unauthorized access detected');
* logger.a`Security alert: ${securityError}`; // Logs: "Security alert: Unauthorized access detected"
* ```
*
* @param strings The template strings
* @param values The values to be interpolated into the template
*/
readonly a: (strings: LogTemplateStringsArray, ...values: unknown[]) => void;
/**
* Logs an emergency-level entry for when the system is unusable (e.g., complete system failure).
*
* Emergency entries are emitted at all logger levels except when the logger level is set to 'off'.
*
* The emergency entry content can be:
*
* - A direct value that is logged as is,
* - A function that returns a value, useful for messages that are expensive to compute,
* - An Error object, which is automatically stringified and added as an additional argument,
* - An object with an `error` property that is logged as a string and added as an additional argument - use this
* whenever logging an error of type unknown.
*
* @example
*
* ```ts
* logger.emergency('System is shutting down due to critical failure');
* logger.emergency(() => `Fatal error in primary system: ${errorDetails}`);
* logger.emergency(new Error('Connection timeout'), error);
* logger.emergency({ error: ['Database connection failed', errorCode] });
* ```
*
* @param input The entry content or function that returns the content
* @param args Additional arguments to include in the log entry
*/
readonly emergency: (input: LogMessage | Error | { error: unknown }, ...args: unknown[]) => void;
/**
* Logs an emergency-level entry using a template string, which is computed at all logger levels except 'off'.
*
* Values in template literals are automatically formatted:
*
* - Error objects will show their message text (e.g., `${new Error('Connection failed')}` becomes 'Connection failed')
* - Objects with a `message` property will display that message
* - Functions are executed and their return values formatted using the rules above
* - All other values are converted to their string representation
*
* @example
*
* ```ts
* // Basic template literal
* logger.em`System is shutting down due to critical failure`;
*
* // With computed values
* logger.em`Fatal error in primary system: ${errorDetails}`;
*
* // With error objects - message is automatically extracted
* const fatalError = new Error('Catastrophic failure in database cluster');
* logger.em`EMERGENCY: ${fatalError}`; // Logs: "EMERGENCY: Catastrophic failure in database cluster"
* ```
*
* @param strings The template strings
* @param values The values to be interpolated into the template
*/
readonly em: (strings: LogTemplateStringsArray, ...values: unknown[]) => void;
/**
* Logs an entry at a specific severity level. This method is useful for dynamically setting the level without having
* to call the specific level method directly.
*
* The entry content can be either a direct value or a function that returns a value. Using a function is recommended
* when the message is expensive to construct, as the function will only be invoked if the entry will actually be
* emitted based on the current logger level.
*
* @example
*
* ```ts
* logger.log('info', 'Operation completed successfully');
* logger.log('warning', () => `High latency detected: ${latency}ms`);
* ```
*
* @param level The severity level for the log entry
* @param message The entry content or function that returns the content
* @param args Additional arguments to include in the log entry
*/
readonly log: (level: LogLevel, message: LogMessage, ...args: unknown[]) => void;
/**
* Flushes any buffered log entries, ensuring they are written to their destination.
*
* This method is optional and may not be available on all logger implementations. It is primarily used by loggers
* that buffer entries for performance reasons (such as batch loggers or file loggers) to force immediate writing of
* pending entries.
*
* The method can be either synchronous or asynchronous depending on the underlying implementation:
*
* - **Synchronous**: Returns `void` when the flush operation completes immediately
* - **Asynchronous**: Returns a `Promise<void>` that resolves when the flush operation completes
*
* @example Handling both sync and async flush
*
* ```ts
* const logger = createSomeLogger();
* logger.info('Important message');
*
* // This approach simple calls the flush without investigating
* // if it's defined, async or sync
* await logger?.flush();
* ```
*
* @example Synchronous flush
*
* ```ts
* import { createConsoleLogLogger } from 'emitnlog/logger';
*
* const logger = createConsoleLogLogger();
* logger.info('Buffered message');
*
* if (logger.flush) {
* logger.flush(); // Synchronous flush
* }
* ```
*
* @example Asynchronous flush
*
* ```ts
* import { createFileLogger } from 'emitnlog/logger/node';
*
* const logger = createFileLogger('app.log');
* logger.info('Buffered message');
*
* if (logger.flush) {
* await logger.flush(); // Asynchronous flush
* }
* ```
*
* @returns Either void for synchronous flush or Promise<void> for asynchronous flush, or undefined if not supported
*/
readonly flush?: () => void | Promise<void>;
/**
* Closes the logger and releases any associated resources.
*
* This method is optional and may not be available on all logger implementations. It should be called when the logger
* is no longer needed to ensure proper cleanup of resources such as file handles, network connections, or other
* system resources.
*
* The method can be either synchronous or asynchronous depending on the underlying implementation:
*
* - **Synchronous**: Returns `void` when the close operation completes immediately
* - **Asynchronous**: Returns a `Promise<void>` that resolves when the close operation completes
*
* After calling `close()`, the logger should not be used for further logging operations. Calling `flush()` before
* `close()` is recommended to ensure all pending log entries are written before cleanup.
*
* @example Handling both sync and async close
*
* ```ts
* const logger = createSomeLogger();
* logger.info('Important message');
*
* // This approach simple calls the flush without investigating
* // if it's defined, async or close
* await logger?.close();
* ```
*
* @example Synchronous close
*
* ```ts
* import { createConsoleLogLogger } from 'emitnlog/logger';
*
* const logger = createConsoleLogLogger();
* logger.info('Final message');
*
* if (logger.close) {
* logger.close(); // Synchronous close
* }
* ```
*
* @example Asynchronous close with file logger
*
* ```ts
* import { createFileLogger } from 'emitnlog/logger/node';
*
* const logger = createFileLogger('app.log');
* logger.info('Final message');
*
* if (logger.close) {
* await logger.close(); // Asynchronous close
* }
* ```
*
* @example A shutdown sequence that precisely investigates if the methods are sync or async.
*
* ```ts
* const logger = createSomeLogger();
* logger.info('Application shutting down');
*
* // Flush pending entries before closing
* if (logger.flush) {
* const flushResult = logger.flush();
* if (flushResult instanceof Promise) {
* await flushResult;
* }
* }
*
* // Close and release resources
* if (logger.close) {
* const closeResult = logger.close();
* if (closeResult instanceof Promise) {
* await closeResult;
* }
* }
* ```
*
* @returns Either void for synchronous close or Promise<void> for asynchronous close, or undefined if not supported
*/
readonly close?: () => void | Promise<void>;
}
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