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import type { Logger, LogLevel, LogMessage } from './definition.ts';
import { shouldEmitEntry } from './level-utils.ts';
import { OFF_LOGGER } from './off-logger.ts';
const prefixSymbol: unique symbol = Symbol.for('@emitnlog/logger/prefix');
const separatorSymbol: unique symbol = Symbol.for('@emitnlog/logger/separator');
const dataSymbol: unique symbol = Symbol.for('@emitnlog/logger/data');
/**
* A specialized logger that prepends a fixed prefix to all log messages.
*
* The PrefixedLogger maintains all the functionality of a standard Logger while ensuring every log message is
* automatically prefixed, making it easier to:
*
* - Categorize logs by component, module, or subsystem
* - Distinguish logs from different parts of your application
* - Create hierarchical logging structures with nested prefixes
*
* The prefix is strongly typed and, in some development environments, is visible on the logger's tooltip.
*/
export interface PrefixedLogger<TPrefix extends string = string, TSeparator extends string = string> extends Logger {
/*
* The prefix or undefined.
*/
readonly [prefixSymbol]: TPrefix | undefined;
/*
* The value used to separate different parts of the prefix, or undefined
*/
readonly [separatorSymbol]: TSeparator | undefined;
/*
* Arbitrary data needed by the prefix logger.
*/
readonly [dataSymbol]: { readonly rootLogger: Logger; readonly messageSeparator: string } | undefined;
}
/**
* Creates a new logger that prepends a specified prefix to all log messages.
*
* This function wraps an existing logger and returns a new logger instance where all log messages are automatically
* prefixed. This is useful for categorizing logs or distinguishing between different components in your application.
* The log level of a prefixed logger is always the same as its underlying logger.
*
* When applied to an already prefixed logger, it preserves the full prefix chain both at runtime and in the type
* system, allowing for strongly-typed nested prefixes.
*
* @example Basic Usage
*
* ```ts
* import { ConsoleLogger, withPrefix } from 'emitnlog/logger';
*
* const logger = new ConsoleLogger();
* const dbLogger = withPrefix(logger, 'DB');
*
* dbLogger.info('Connected to database');
* // Output: "DB: Connected to database"
* ```
*
* @example Template Literals
*
* ```ts
* const dbLogger = withPrefix(logger, 'DB');
* const queryTime = 42;
*
* // Prefix is maintained with template literals
* dbLogger.d`Query executed in ${queryTime}ms`;
* // Output: "DB: Query executed in 42ms"
* ```
*
* @example Nested Prefixes
*
* ```ts
* const dbLogger = withPrefix(logger, 'DB');
* const userDbLogger = withPrefix(dbLogger, 'users');
* // Type of userDbLogger is PrefixedLogger<'DB.users'>
*
* const userId = 123;
* userDbLogger.w`User not found: ${userId}`;
* // Output: "DB.users: User not found: 123"
* ```
*
* @example Error Handling
*
* ```ts
* const dbLogger = withPrefix(logger, 'DB');
*
* // Errors maintain their original objects while prefixing the message
* const error = new Error('Connection failed');
* dbLogger.error(error);
* // Output: "DB: Connection failed" (with error object preserved in args)
*
* // Using args() for additional context
* dbLogger.args(error, { connectionId: 'conn_123' }).e`Database operation failed`;
* ```
*
* @example Custom Separators
*
* ```ts
* // Custom prefix separator
* const apiLogger = withPrefix(logger, 'API', { prefixSeparator: '/' });
* const v1Logger = withPrefix(apiLogger, 'v1');
* v1Logger.info('Request processed');
* // Output: "API/v1: Request processed"
*
* // Custom message separator
* const compactLogger = withPrefix(logger, 'SYS', { messageSeparator: ' | ' });
* compactLogger.info('System ready');
* // Output: "SYS | System ready"
* ```
*
* @example Fallback Prefix
*
* ```ts
* // Add a fallback prefix when the logger isn't already prefixed
* const serviceLogger = withPrefix(logger, 'UserService', { fallbackPrefix: 'APP' });
* serviceLogger.info('Service started');
* // Output: "APP.UserService: Service started"
*
* // If applied to an already prefixed logger, fallback is ignored
* const dbLogger = withPrefix(logger, 'DB');
* const userDbLogger = withPrefix(dbLogger, 'UserService', {
* fallbackPrefix: 'APP', // This is ignored
* });
* userDbLogger.info('Service started');
* // Output: "DB.UserService: Service started"
* ```
*
* @example Level Filtering
*
* ```ts
* const dbLogger = withPrefix(logger, 'DB');
*
* // Level filtering works the same as the underlying logger
* dbLogger.level = 'warning';
* dbLogger.d`This debug message won't be logged`;
* dbLogger.w`This warning will be logged`;
* // Output: "DB: This warning will be logged"
* ```
*
* **Tip:** When supported by your development environment, hovering over a prefixed logger variable shows the prefix in
* the tooltip due to the type parameter, making it easier to identify which prefix is being used.
*
* @param logger The base logger to wrap with prefix functionality
* @param prefix The prefix to prepend to all log messages
* @param options Optional configuration for the logger behavior
* @returns A new logger with the specified prefix, or the OFF_LOGGER if the input logger is OFF_LOGGER
*/
export const withPrefix = <
TLogger extends Logger,
const TPrefix extends string,
const TSeparator extends string = '.',
const TFallbackPrefix extends string | undefined = undefined,
>(
logger: TLogger,
prefix: TPrefix,
options?: {
/**
* The separator to use between an existing prefix and the prefix passed to this method. Defaults to '.'.
*/
readonly prefixSeparator?: TSeparator;
/**
* The separator to use between the prefix and the log message. Defaults to ': '.
*/
readonly messageSeparator?: string;
/**
* The fallback prefix to use if the logger is not already a prefixed logger.
*/
readonly fallbackPrefix?: TFallbackPrefix;
},
): WithPrefixResult<TLogger, TPrefix, TSeparator, TFallbackPrefix> => {
if (logger === OFF_LOGGER) {
return OFF_LOGGER as unknown as WithPrefixResult<TLogger, TPrefix, TSeparator, TFallbackPrefix>;
}
let prefixSeparator: TSeparator;
let messageSeparator: string;
const data = inspectPrefixedLogger(logger);
if (data) {
logger = data.rootLogger as TLogger;
prefixSeparator = data.separator as TSeparator;
messageSeparator = data.messageSeparator;
prefix = (prefix ? `${data.prefix}${prefixSeparator}${prefix}` : data.prefix) as TPrefix;
} else {
prefixSeparator = (options?.prefixSeparator ?? '.') as TSeparator;
messageSeparator = options?.messageSeparator ?? ': ';
if (options?.fallbackPrefix) {
prefix = (prefix ? `${options.fallbackPrefix}${prefixSeparator}${prefix}` : options.fallbackPrefix) as TPrefix;
}
}
const prefixedLogger: PrefixedLogger<TPrefix, TSeparator> = {
[prefixSymbol]: prefix,
[separatorSymbol]: prefixSeparator,
[dataSymbol]: { rootLogger: logger, messageSeparator },
get level() {
return logger.level;
},
set level(value: LogLevel | 'off') {
logger.level = value;
},
args(...args: unknown[]) {
logger.args(...args);
return prefixedLogger;
},
trace(message: LogMessage, ...args: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'trace')) {
logger.trace(toMessageProvider(prefixedLogger, message), ...args);
}
},
t(strings: TemplateStringsArray, ...values: unknown[]) {
if (shouldEmitEntry(logger.level, 'trace')) {
logger.t(prefixTemplateString(prefixedLogger, strings), ...values);
}
},
debug(message: LogMessage, ...args: unknown[]) {
if (shouldEmitEntry(logger.level, 'debug')) {
logger.debug(toMessageProvider(prefixedLogger, message), ...args);
}
},
d(strings: TemplateStringsArray, ...values: unknown[]) {
if (shouldEmitEntry(logger.level, 'debug')) {
logger.d(prefixTemplateString(prefixedLogger, strings), ...values);
}
},
info(message: LogMessage, ...args: unknown[]) {
if (shouldEmitEntry(logger.level, 'info')) {
logger.info(toMessageProvider(prefixedLogger, message), ...args);
}
},
i(strings: TemplateStringsArray, ...values: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'info')) {
logger.i(prefixTemplateString(prefixedLogger, strings), ...values);
}
},
notice(message: LogMessage, ...args: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'notice')) {
logger.notice(toMessageProvider(prefixedLogger, message), ...args);
}
},
n(strings: TemplateStringsArray, ...values: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'notice')) {
logger.n(prefixTemplateString(prefixedLogger, strings), ...values);
}
},
warning(message: LogMessage, ...args: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'warning')) {
logger.warning(toMessageProvider(prefixedLogger, message), ...args);
}
},
w(strings: TemplateStringsArray, ...values: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'warning')) {
logger.w(prefixTemplateString(prefixedLogger, strings), ...values);
}
},
error(error: LogMessage | Error | { error: unknown }, ...args: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'error')) {
if (error instanceof Error) {
logger.error(toMessageProvider(prefixedLogger, error.message), error, ...args);
} else if (error && typeof error === 'object' && 'error' in error) {
logger.error(toMessageProvider(prefixedLogger, String(error.error)), error, ...args);
} else {
logger.error(toMessageProvider(prefixedLogger, error as LogMessage), ...args);
}
}
},
e(strings: TemplateStringsArray, ...values: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'error')) {
logger.e(prefixTemplateString(prefixedLogger, strings), ...values);
}
},
critical(message: LogMessage, ...args: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'critical')) {
logger.critical(toMessageProvider(prefixedLogger, message), ...args);
}
},
c(strings: TemplateStringsArray, ...values: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'critical')) {
logger.c(prefixTemplateString(prefixedLogger, strings), ...values);
}
},
alert(message: LogMessage, ...args: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'alert')) {
logger.alert(toMessageProvider(prefixedLogger, message), ...args);
}
},
a(strings: TemplateStringsArray, ...values: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'alert')) {
logger.a(prefixTemplateString(prefixedLogger, strings), ...values);
}
},
emergency(message: LogMessage, ...args: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'emergency')) {
logger.emergency(toMessageProvider(prefixedLogger, message), ...args);
}
},
em(strings: TemplateStringsArray, ...values: unknown[]) {
Eif (shouldEmitEntry(logger.level, 'emergency')) {
logger.em(prefixTemplateString(prefixedLogger, strings), ...values);
}
},
log(level: LogLevel, message: LogMessage, ...args: unknown[]) {
Eif (shouldEmitEntry(logger.level, level)) {
logger.log(level, toMessageProvider(prefixedLogger, message), ...args);
}
},
};
return prefixedLogger as unknown as WithPrefixResult<TLogger, TPrefix, TSeparator, TFallbackPrefix>;
};
/**
* Appends a prefix to an existing prefixed logger, creating a hierarchical prefix structure.
*
* This utility function is specifically designed for use with already prefixed loggers. It extends the existing prefix
* chain by appending a new prefix segment, using the same separator that was used in the original logger.
*
* Unlike `withPrefix`, this function has a simplified API that doesn't expose configuration options since it inherits
* all settings (separators, etc.) from the existing prefixed logger.
*
* @example Basic Appending
*
* ```ts
* import { consoleLogger, withPrefix, appendPrefix } from 'emitnlog/logger';
*
* const logger = new ConsoleLogger();
* const dbLogger = withPrefix(logger, 'DB');
* const userDbLogger = appendPrefix(dbLogger, 'users');
*
* userDbLogger.info('User created successfully');
* // Output: "DB.users: User created successfully"
* ```
*
* @example Multiple Levels of Nesting
*
* ```ts
* const serviceLogger = withPrefix(logger, 'UserService');
* const repositoryLogger = appendPrefix(serviceLogger, 'Repository');
* const cacheLogger = appendPrefix(repositoryLogger, 'Cache');
*
* cacheLogger.d`Cache hit for key: user_123`;
* // Output: "UserService.Repository.Cache: Cache hit for key: user_123"
* ```
*
* @example Preserving Custom Separators
*
* ```ts
* // Original logger with custom separator
* const apiLogger = withPrefix(logger, 'API', { prefixSeparator: '/' });
* const v1Logger = appendPrefix(apiLogger, 'v1');
* const usersLogger = appendPrefix(v1Logger, 'users');
*
* usersLogger.info('Processing user request');
* // Output: "API/v1/users: Processing user request"
* ```
*
* @example Type Safety
*
* ```ts
* const dbLogger = withPrefix(logger, 'DB');
* const userLogger = appendPrefix(dbLogger, 'User');
* // Type of userLogger is PrefixedLogger<'DB.User', '.'>
*
* // TypeScript knows the exact prefix structure
* const profileLogger = appendPrefix(userLogger, 'Profile');
* // Type of profileLogger is PrefixedLogger<'DB.User.Profile', '.'>
* ```
*
* @param logger The prefixed logger to append the prefix to
* @param prefix The prefix segment to append to the existing prefix chain
* @returns A new logger with the appended prefix, maintaining all original configuration
*/
export const appendPrefix = <
const TCurrentPrefix extends string,
const TSeparator extends string,
const TLogger extends PrefixedLogger<TCurrentPrefix, TSeparator>,
const TPrefix extends string,
>(
logger: TLogger,
prefix: TPrefix,
): WithPrefixResult<TLogger, TPrefix, TSeparator> => withPrefix(logger, prefix);
/**
* Creates a prefixed logger with a completely new prefix, ignoring any existing prefix on the input logger.
*
* This utility function extracts the root (non-prefixed) logger from the input and applies a fresh prefix to it,
* effectively "resetting" the prefix chain. This is useful when you want to create a new prefix hierarchy without
* inheriting the existing prefix structure.
*
* Unlike `withPrefix`, which appends to existing prefixes, `resetPrefix` always starts with a clean slate.
*
* @example Basic Reset
*
* ```ts
* import { ConsoleLogger, withPrefix, resetPrefix } from 'emitnlog/logger';
*
* const logger = new ConsoleLogger();
* const dbLogger = withPrefix(logger, 'DB');
* const userDbLogger = withPrefix(dbLogger, 'users'); // Prefix: "DB.users"
*
* // Reset to a completely new prefix
* const apiLogger = resetPrefix(userDbLogger, 'API'); // Prefix: "API" (not "DB.users.API")
*
* apiLogger.info('API server started');
* // Output: "API: API server started"
* ```
*
* @example Switching Context
*
* ```ts
* // Start with a deeply nested logger
* const serviceLogger = withPrefix(logger, 'UserService');
* const repoLogger = appendPrefix(serviceLogger, 'Repository');
* const cacheLogger = appendPrefix(repoLogger, 'Cache'); // Prefix: "UserService.Repository.Cache"
*
* // Switch to a completely different context
* const authLogger = resetPrefix(cacheLogger, 'Auth'); // Prefix: "Auth"
* const tokenLogger = appendPrefix(authLogger, 'Token'); // Prefix: "Auth.Token"
*
* tokenLogger.d`Token validated successfully`;
* // Output: "Auth.Token: Token validated successfully"
* ```
*
* @example Custom Configuration
*
* ```ts
* const existingLogger = withPrefix(logger, 'OldPrefix');
*
* // Reset with custom separators
* const newLogger = resetPrefix(existingLogger, 'NewPrefix', { prefixSeparator: '/', messageSeparator: ' >> ' });
*
* const subLogger = appendPrefix(newLogger, 'SubModule');
* subLogger.info('Module initialized');
* // Output: "NewPrefix/SubModule >> Module initialized"
* ```
*
* @example Reusing Root Logger
*
* ```ts
* // Multiple loggers sharing the same root but with different prefixes
* const dbLogger = withPrefix(logger, 'DB');
* const complexDbLogger = appendPrefix(dbLogger, 'Complex'); // Prefix: "DB.Complex"
*
* // Create parallel hierarchies from the same root
* const cacheLogger = resetPrefix(complexDbLogger, 'Cache'); // Uses same root as dbLogger
* const metricsLogger = resetPrefix(complexDbLogger, 'Metrics'); // Uses same root as dbLogger
*
* cacheLogger.info('Cache warmed up'); // Output: "Cache: Cache warmed up"
* metricsLogger.info('Metrics collected'); // Output: "Metrics: Metrics collected"
* ```
*
* @param logger The logger to extract the root logger from and apply a new prefix to
* @param prefix The new prefix to apply (completely replacing any existing prefix)
* @param options Optional configuration for the new prefixed logger
* @returns A new logger with the specified prefix, using the root logger from the input
*/
export const resetPrefix = <const TPrefix extends string, const TSeparator extends string = '.'>(
logger: Logger,
prefix: TPrefix,
options?: {
/**
* The separator to use between an existing prefix and the prefix passed to this method. Defaults to '.'.
*/
readonly prefixSeparator?: TSeparator;
/**
* The separator to use between the prefix and the log message. Defaults to ': '.
*/
readonly messageSeparator?: string;
},
): WithPrefixResult<Logger, TPrefix, TSeparator> => {
const data = inspectPrefixedLogger(logger);
if (data) {
logger = data.rootLogger;
}
return withPrefix(logger, prefix, options);
};
export const isPrefixedLogger = (logger: Logger | undefined | null): logger is PrefixedLogger =>
isNotNullable(logger) && prefixSymbol in logger && typeof logger[prefixSymbol] === 'string' && dataSymbol in logger;
export const inspectPrefixedLogger = (
logger: Logger,
):
| {
readonly rootLogger: Logger;
readonly prefix: string;
readonly separator: string;
readonly messageSeparator: string;
}
| undefined =>
isPrefixedLogger(logger)
? {
rootLogger: logger[dataSymbol]!.rootLogger,
prefix: logger[prefixSymbol]!,
separator: logger[separatorSymbol]!,
messageSeparator: logger[dataSymbol]!.messageSeparator,
}
: undefined;
const prefixTemplateString = (prefixLogger: PrefixedLogger, strings: TemplateStringsArray): TemplateStringsArray => {
const prefix = prefixLogger[prefixSymbol]!;
const messageSeparator = prefixLogger[dataSymbol]!.messageSeparator;
const newStrings = Array.from(strings);
newStrings[0] = `${prefix}${messageSeparator}${newStrings[0]}`;
const prefixedStrings = Object.assign(newStrings, { raw: Array.from(strings.raw) });
prefixedStrings.raw[0] = `${prefix}${messageSeparator}${prefixedStrings.raw[0]}`;
return prefixedStrings as unknown as TemplateStringsArray;
};
const toMessageProvider = (prefixLogger: PrefixedLogger, message: LogMessage) => () => {
const messageString = typeof message === 'function' ? message() : message;
const messageSeparator = prefixLogger[dataSymbol]!.messageSeparator;
return `${prefixLogger[prefixSymbol]}${messageSeparator}${messageString}`;
};
type WithPrefixResult<
TLogger extends Logger,
TNewPrefix extends string,
TSeparator extends string = '.',
TFallbackPrefix extends string | undefined = undefined,
> =
TLogger extends PrefixedLogger<infer TPrevPrefix, infer TPrevSeparator>
? PrefixedLogger<`${TPrevPrefix}${TPrevSeparator}${TNewPrefix}`, TPrevSeparator>
: TFallbackPrefix extends string
? TNewPrefix extends ''
? PrefixedLogger<`${TFallbackPrefix}`, TSeparator>
: PrefixedLogger<`${TFallbackPrefix}${TSeparator}${TNewPrefix}`, TSeparator>
: PrefixedLogger<TNewPrefix, TSeparator>;
|