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import { isNotNullable } from '../utils/common/is-not-nullable.ts';
import type { Logger, LogMessage, LogTemplateStringsArray } from './definition.ts';
import { BaseLogger } from './implementation/base-logger.ts';
import { OFF_LOGGER } from './off-logger.ts';
export type PrefixedLogger<
TPrefix extends string = string,
TSeparator extends string | undefined = undefined,
TLogger extends Logger = Logger,
> =
TLogger extends PrefixedWithSeparator<infer TBaseLogger, infer TPrevPrefix, infer TPrevSeparator>
? PrefixedWithSeparator<
TBaseLogger,
`${TPrevPrefix}${TPrevSeparator}${TPrefix}`,
TSeparator extends undefined | '' ? TPrevSeparator : TSeparator
>
: TLogger extends Prefixed<infer TBaseLogger, string>
? TSeparator extends undefined | ''
? Prefixed<TBaseLogger, ResolvePrefix<TLogger, TPrefix, TSeparator>>
: PrefixedWithSeparator<
TBaseLogger,
ResolvePrefix<TLogger, TPrefix, TSeparator>,
TSeparator extends undefined ? '.' : TSeparator
>
: TPrefix extends '' | undefined
? TLogger
: TSeparator extends undefined | ''
? Prefixed<TLogger, TPrefix>
: PrefixedWithSeparator<TLogger, TPrefix, TSeparator extends undefined ? '.' : TSeparator>;
/**
* 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 { createConsoleLogLogger, withPrefix } from 'emitnlog/logger';
*
* const logger = createConsoleLogLogger('info');
* const dbLogger = withPrefix(logger, 'DB');
*
* dbLogger.i`Connected to database`;
* // Output: "DB: Connected to database"
* ```
*
* @example Template Literals
*
* ```ts
* import { withPrefix } from 'emitnlog/logger';
*
* 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
* import { withPrefix } from 'emitnlog/logger';
*
* 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
* import { withPrefix } from 'emitnlog/logger';
*
* 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
* import { withPrefix } from 'emitnlog/logger';
*
* // Custom prefix separator
* const apiLogger = withPrefix(logger, 'API', { prefixSeparator: '/' });
* const v1Logger = withPrefix(apiLogger, 'v1');
* v1Logger.i`Request processed`;
* // Output: "API/v1: Request processed"
*
* // Custom message separator
* const compactLogger = withPrefix(logger, 'SYS', { messageSeparator: ' | ' });
* compactLogger.i`System ready`;
* // Output: "SYS | System ready"
* ```
*
* @example Fallback Prefix
*
* ```ts
* import { withPrefix } from 'emitnlog/logger';
*
* // Add a fallback prefix when the logger isn't already prefixed
* const serviceLogger = withPrefix(logger, 'UserService', { fallbackPrefix: 'APP' });
* serviceLogger.i`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.i`Service started`;
* // Output: "DB.UserService: Service started"
* ```
*
* @example Level Filtering
*
* ```ts
* import { createConsoleLogLogger, withPrefix } from 'emitnlog/logger';
*
* // Underlying logger controls filtering
* const base = createConsoleLogLogger('warning');
* const dbLogger = withPrefix(base, 'DB');
*
* 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 = <
const TLogger extends Logger,
const TPrefix extends string,
const TSeparator extends string | undefined = undefined,
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;
},
): PrefixedLogger<ResolvePrefixWithFallback<TLogger, TPrefix, TSeparator, TFallbackPrefix>, TSeparator, TLogger> => {
if (logger === OFF_LOGGER) {
return OFF_LOGGER as unknown as PrefixedLogger<
ResolvePrefixWithFallback<TLogger, TPrefix, TSeparator, TFallbackPrefix>,
TSeparator,
TLogger
>;
}
let prefixSeparator: TSeparator;
let messageSeparator: string;
const data = inspectPrefixedLogger(logger);
if (data) {
logger = data.rootLogger as TLogger;
prefixSeparator = (options?.prefixSeparator || data.separator) as TSeparator;
messageSeparator = data.messageSeparator;
prefix = (prefix ? `${data.prefix}${data.separator || '.'}${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;
}
}
if (!prefix) {
return logger as unknown as PrefixedLogger<
ResolvePrefixWithFallback<TLogger, TPrefix, TSeparator, TFallbackPrefix>,
TSeparator,
TLogger
>;
}
const prefixedLogger = createPrefixedLogger(logger, prefix, prefixSeparator, messageSeparator);
return prefixedLogger as unknown as PrefixedLogger<
ResolvePrefixWithFallback<TLogger, TPrefix, TSeparator, TFallbackPrefix>,
TSeparator,
TLogger
>;
};
const createPrefixedLogger = <TPrefix extends string = string, TSeparator extends string | undefined = undefined>(
rootLogger: Logger,
prefix: TPrefix,
prefixSeparator: TSeparator,
messageSeparator: string,
): Logger => {
let pendingArgs: unknown[] = [];
const consumePendingArgs = (): readonly unknown[] | undefined => {
if (!pendingArgs.length) {
return undefined;
}
const args = pendingArgs;
pendingArgs = [];
return args;
};
const runLogOperation = (internalLogger: InternalPrefixedLogger, operation: (logger: Logger) => void) => {
const logger = internalLogger[rootLoggerSymbol];
const currentArgs = consumePendingArgs();
if (currentArgs) {
logger.args(...currentArgs);
}
operation(logger);
};
const internalLogger: InternalPrefixedLogger = {
[prefixSymbol]: prefix,
[separatorSymbol]: prefixSeparator,
[messageSeparatorSymbol]: messageSeparator,
[rootLoggerSymbol]: rootLogger,
get level() {
return internalLogger[rootLoggerSymbol].level;
},
args: (...args) => {
pendingArgs.push(...args);
return internalLogger;
},
trace: (message, ...args) => {
runLogOperation(internalLogger, (logger) => logger.trace(toMessageProvider(internalLogger, message), ...args));
},
t: (strings, ...values) => {
runLogOperation(internalLogger, (logger) => logger.t(toTemplateProvider(internalLogger, strings), ...values));
},
debug: (message, ...args) => {
runLogOperation(internalLogger, (logger) => logger.debug(toMessageProvider(internalLogger, message), ...args));
},
d: (strings, ...values) => {
runLogOperation(internalLogger, (logger) => logger.d(toTemplateProvider(internalLogger, strings), ...values));
},
info: (message, ...args) => {
runLogOperation(internalLogger, (logger) => logger.info(toMessageProvider(internalLogger, message), ...args));
},
i: (strings, ...values) => {
runLogOperation(internalLogger, (logger) => logger.i(toTemplateProvider(internalLogger, strings), ...values));
},
notice: (message, ...args) => {
runLogOperation(internalLogger, (logger) => logger.notice(toMessageProvider(internalLogger, message), ...args));
},
n: (strings, ...values) => {
runLogOperation(internalLogger, (logger) => logger.n(toTemplateProvider(internalLogger, strings), ...values));
},
warning: (input, ...args) => {
runLogOperation(internalLogger, (logger) => {
const converted = toErrorInput(internalLogger, input, args);
logger.warning(converted.message, ...converted.args);
});
},
w: (strings, ...values) => {
runLogOperation(internalLogger, (logger) => logger.w(toTemplateProvider(internalLogger, strings), ...values));
},
error: (input, ...args) => {
runLogOperation(internalLogger, (logger) => {
const converted = toErrorInput(internalLogger, input, args);
logger.error(converted.message, ...converted.args);
});
},
e: (strings, ...values) => {
runLogOperation(internalLogger, (logger) => logger.e(toTemplateProvider(internalLogger, strings), ...values));
},
critical: (input, ...args) => {
runLogOperation(internalLogger, (logger) => {
const converted = toErrorInput(internalLogger, input, args);
logger.critical(converted.message, ...converted.args);
});
},
c: (strings, ...values) => {
runLogOperation(internalLogger, (logger) => logger.c(toTemplateProvider(internalLogger, strings), ...values));
},
alert: (input, ...args) => {
runLogOperation(internalLogger, (logger) => {
const converted = toErrorInput(internalLogger, input, args);
logger.alert(converted.message, ...converted.args);
});
},
a: (strings, ...values) => {
runLogOperation(internalLogger, (logger) => logger.a(toTemplateProvider(internalLogger, strings), ...values));
},
emergency: (input, ...args) => {
runLogOperation(internalLogger, (logger) => {
const converted = toErrorInput(internalLogger, input, args);
logger.emergency(converted.message, ...converted.args);
});
},
em: (strings, ...values) => {
runLogOperation(internalLogger, (logger) => logger.em(toTemplateProvider(internalLogger, strings), ...values));
},
log: (level, message, ...args) => {
runLogOperation(internalLogger, (logger) =>
logger.log(level, toMessageProvider(internalLogger, message), ...args),
);
},
flush: rootLogger.flush ? () => internalLogger[rootLoggerSymbol].flush?.() : undefined,
close: rootLogger.close ? () => internalLogger[rootLoggerSymbol].close?.() : undefined,
} as const;
return internalLogger;
};
const toMessageProvider = (prefixLogger: InternalPrefixedLogger, message: LogMessage) => (): string => {
if (typeof message === 'function') {
message = message();
}
return `${prefixLogger[prefixSymbol]}${prefixLogger[messageSeparatorSymbol]}${message}`;
};
const toErrorInput = (
prefixLogger: InternalPrefixedLogger,
input: LogMessage | Error | { error: unknown },
args: readonly unknown[],
): { readonly message: LogMessage; readonly args: readonly unknown[] } => {
const logger = prefixLogger[rootLoggerSymbol];
const converted = BaseLogger.convertErrorInput(logger, input, args);
return { message: toMessageProvider(prefixLogger, converted.message), args: converted.args };
};
const toTemplateProvider =
(prefixLogger: InternalPrefixedLogger, strings: LogTemplateStringsArray) => (): TemplateStringsArray => {
Iif (typeof strings === 'function') {
strings = strings();
}
const prefix = `${prefixLogger[prefixSymbol]}${prefixLogger[messageSeparatorSymbol]}`;
const newStrings = Array.from(strings);
newStrings[0] = `${prefix}${newStrings[0]}`;
const prefixedStrings = Object.assign(newStrings, { raw: Array.from(strings.raw) });
prefixedStrings.raw[0] = `${prefix}${prefixedStrings.raw[0]}`;
return prefixedStrings;
};
/**
* 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 { createConsoleLogLogger, resetPrefix, withPrefix } from 'emitnlog/logger';
*
* const logger = createConsoleLogLogger('info');
* 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.i`API server started`;
* // Output: "API: API server started"
* ```
*
* @example Switching Context
*
* ```ts
* import { resetPrefix, withPrefix } from 'emitnlog/logger';
*
* // Start with a deeply nested logger
* const serviceLogger = withPrefix(logger, 'UserService');
* const repoLogger = withPrefix(serviceLogger, 'Repository');
* const cacheLogger = withPrefix(repoLogger, 'Cache'); // Prefix: "UserService.Repository.Cache"
*
* // Switch to a completely different context
* const authLogger = resetPrefix(cacheLogger, 'Auth'); // Prefix: "Auth"
* const tokenLogger = withPrefix(authLogger, 'Token'); // Prefix: "Auth.Token"
*
* tokenLogger.d`Token validated successfully`;
* // Output: "Auth.Token: Token validated successfully"
* ```
*
* @example Custom Configuration
*
* ```ts
* import { resetPrefix, withPrefix } from 'emitnlog/logger';
*
* const existingLogger = withPrefix(logger, 'OldPrefix');
*
* // Reset with custom separators
* const newLogger = resetPrefix(existingLogger, 'NewPrefix', { prefixSeparator: '/', messageSeparator: ' >> ' });
*
* const subLogger = withPrefix(newLogger, 'SubModule');
* subLogger.i`Module initialized`;
* // Output: "NewPrefix/SubModule >> Module initialized"
* ```
*
* @example Reusing Root Logger
*
* ```ts
* import { resetPrefix, withPrefix } from 'emitnlog/logger';
*
* // Multiple loggers sharing the same root but with different prefixes
* const dbLogger = withPrefix(logger, 'DB');
* const complexDbLogger = withPrefix(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.i`Cache warmed up`; // Output: "Cache: Cache warmed up"
* metricsLogger.i`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 = '.',
const TLogger extends Logger = Logger,
>(
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;
},
) => {
const data = inspectPrefixedLogger(logger);
if (data) {
logger = data.rootLogger as TLogger;
}
return withPrefix(logger as Logger, prefix, options);
};
/**
* Checks if a logger is a prefixed logger.
*
* @param logger The logger to check.
* @returns True if the logger is a prefixed logger, false otherwise.
*/
export const isPrefixedLogger = (logger: Logger | undefined | null): logger is PrefixedLogger =>
isInternalPrefixedLogger(logger);
export type PrefixInformation = {
readonly rootLogger: Logger;
readonly prefix: string;
readonly separator: string | undefined;
readonly messageSeparator: string;
};
/**
* Inspects a logger to get its prefix information.
*
* @param logger The logger to inspect.
* @returns The prefix information, or undefined if the logger is not a prefixed logger.
*/
export const inspectPrefixedLogger = (logger: Logger): PrefixInformation | undefined =>
isInternalPrefixedLogger(logger)
? {
rootLogger: logger[rootLoggerSymbol],
prefix: logger[prefixSymbol],
separator: logger[separatorSymbol],
messageSeparator: logger[messageSeparatorSymbol],
}
: undefined;
/**
* Important: this is an advanced utility, meant for logger implementors.
*
* Injects the prefix information from a source logger into a target logger so that the target is handled as a prefixed
* logger by other utilities.
*
* Use it carefully:
*
* - This method modifies `target` so that object must not be read-only.
* - This method does not cause `target` to write out the prefixed messages: this MUST be already handled by `target`
*
* @param source A logger
* @param target A logger to inject the prefix information into
* @returns If source is prefixed, the modified target logger. Otherwise the unmodified target.
*/
export const injectPrefixInformation = <P extends PrefixedLogger>(source: P, target: Logger): P => {
const prefixInformation = inspectPrefixedLogger(source);
Eif (prefixInformation) {
(target as unknown as Writable<InternalPrefixedLogger>)[rootLoggerSymbol] = prefixInformation.rootLogger;
(target as unknown as Writable<InternalPrefixedLogger>)[prefixSymbol] = prefixInformation.prefix;
(target as unknown as Writable<InternalPrefixedLogger>)[separatorSymbol] = prefixInformation.separator;
(target as unknown as Writable<InternalPrefixedLogger>)[messageSeparatorSymbol] =
prefixInformation.messageSeparator;
}
return target as unknown as P;
};
/**
* Important: this is an advanced utility, meant for logger implementors.
*
* If needed, creates a new prefixed logger that wraps the root-most logger in a prefix chain with the specified wrapper
* so that the wrapping is applied after all prefixes are combined.
*
* @param source A logger
* @param wrapper The wrapper to be applied
* @returns If logger is prefixed, the new prefixed logger with the root-most logger wrapped. Otherwise the wrapped
* version of the specified logger.
*/
export const handlePrefixWrapping = (logger: Logger, wrapper: (logger: Logger) => Logger): Logger => {
const prefixInformation = inspectPrefixedLogger(logger);
if (prefixInformation) {
const wrapped = handlePrefixWrapping(prefixInformation.rootLogger, wrapper);
logger = createPrefixedLogger(
wrapped,
prefixInformation.prefix,
prefixInformation.separator,
prefixInformation.messageSeparator,
);
} else {
logger = wrapper(logger);
}
return logger;
};
declare const tag: unique symbol;
type PrefixContainer<Token> = { readonly [tag]: Token };
type ResolvePrefixWithFallback<
TLogger extends Logger,
TNewPrefix extends string,
TSeparator extends string | undefined = undefined,
TFallbackPrefix extends string | undefined = undefined,
> =
TLogger extends Prefixed<Logger, string>
? TNewPrefix
: TFallbackPrefix extends '' | undefined
? TNewPrefix
: TNewPrefix extends ''
? `${TFallbackPrefix}`
: `${TFallbackPrefix}${UseSeparator<TSeparator>}${TNewPrefix}`;
type ResolvePrefix<
TLogger extends Logger,
TNewPrefix extends string,
TSeparator extends string | undefined = undefined,
TFallbackPrefix extends string | undefined = undefined,
> =
TLogger extends Prefixed<Logger, infer TPrevPrefix>
? TNewPrefix extends ''
? `${TPrevPrefix}`
: `${TPrevPrefix}${UseSeparator<TSeparator>}${TNewPrefix}`
: TFallbackPrefix extends '' | undefined
? TNewPrefix
: TNewPrefix extends ''
? `${TFallbackPrefix}`
: `${TFallbackPrefix}${UseSeparator<TSeparator>}${TNewPrefix}`;
type Prefix<TValue extends string> = PrefixContainer<{ 'emitnlog.prefix': TValue }>;
type Separator<TValue extends string> = PrefixContainer<{ 'emitnlog.separator': TValue }>;
type Prefixed<TLogger extends Logger, TPrefix extends string> = TLogger & Prefix<TPrefix>;
type PrefixedWithSeparator<TLogger extends Logger, TPrefix extends string, TSeparator extends string> = TLogger &
Prefix<TPrefix> &
Separator<TSeparator>;
type UseSeparator<TValue extends string | undefined, TDefault extends string = '.'> = TValue extends undefined | ''
? TDefault
: TValue;
const prefixSymbol: unique symbol = Symbol.for('@emitnlog/logger/prefix');
const separatorSymbol: unique symbol = Symbol.for('@emitnlog/logger/separator');
const messageSeparatorSymbol: unique symbol = Symbol.for('@emitnlog/logger/messageSeparator');
const rootLoggerSymbol: unique symbol = Symbol.for('@emitnlog/logger/rootLogger');
type InternalPrefixedLogger = Logger & {
readonly [prefixSymbol]: string;
readonly [separatorSymbol]: string | undefined;
readonly [messageSeparatorSymbol]: string;
readonly [rootLoggerSymbol]: Logger;
};
const isInternalPrefixedLogger = (logger: Logger | undefined | null): logger is InternalPrefixedLogger =>
isNotNullable(logger) &&
prefixSymbol in logger &&
typeof logger[prefixSymbol] === 'string' &&
separatorSymbol in logger &&
(typeof logger[separatorSymbol] === 'string' || logger[separatorSymbol] === undefined) &&
rootLoggerSymbol in logger &&
logger[rootLoggerSymbol] !== undefined &&
messageSeparatorSymbol in logger &&
typeof logger[messageSeparatorSymbol] === 'string';
|