A minimal lightweight logging for JavaScript, adding reliable log level methods to wrap any available console.log methods.

/*! loglevel - v1.6.6 - https://github.com/pimterry/loglevel - (c) 2019 Tim Perry - licensed MIT */
(function(root, definition) {
    "use strict";
    if (typeof define === "function" && define.amd) {
        define(definition);
    } else if (typeof module === "object" && module.exports) {
        module.exports = definition();
    } else {
        root.log = definition();
    }
})(this, function() {
    "use strict";

    // Slightly dubious tricks to cut down minimized file size
    var noop = function() {};
    var undefinedType = "undefined";
    var isIE =
        typeof window !== undefinedType &&
        /Trident\/|MSIE /.test(window.navigator.userAgent);

    var logMethods = ["trace", "debug", "info", "warn", "error"];

    // Cross-browser bind equivalent that works at least back to IE6
    function bindMethod(obj, methodName) {
        var method = obj[methodName];
        if (typeof method.bind === "function") {
            return method.bind(obj);
        } else {
            try {
                return Function.prototype.bind.call(method, obj);
            } catch (e) {
                // Missing bind shim or IE8 + Modernizr, fallback to wrapping
                return function() {
                    return Function.prototype.apply.apply(method, [
                        obj,
                        arguments
                    ]);
                };
            }
        }
    }

    // Trace() doesn't print the message in IE, so for that case we need to wrap it
    function traceForIE() {
        if (console.log) {
            if (console.log.apply) {
                console.log.apply(console, arguments);
            } else {
                // In old IE, native console methods themselves don't have apply().
                Function.prototype.apply.apply(console.log, [
                    console,
                    arguments
                ]);
            }
        }
        if (console.trace) console.trace();
    }

    // Build the best logging method possible for this env
    // Wherever possible we want to bind, not wrap, to preserve stack traces
    function realMethod(methodName) {
        if (methodName === "debug") {
            methodName = "log";
        }

        if (typeof console === undefinedType) {
            return false; // No method possible, for now - fixed later by enableLoggingWhenConsoleArrives
        } else if (methodName === "trace" && isIE) {
            return traceForIE;
        } else if (console[methodName] !== undefined) {
            return bindMethod(console, methodName);
        } else if (console.log !== undefined) {
            return bindMethod(console, "log");
        } else {
            return noop;
        }
    }

    // These private functions always need `this` to be set properly

    function replaceLoggingMethods(level, loggerName) {
        /*jshint validthis:true */
        for (var i = 0; i < logMethods.length; i++) {
            var methodName = logMethods[i];
            this[methodName] =
                i < level
                    ? noop
                    : this.methodFactory(methodName, level, loggerName);
        }

        // Define log.log as an alias for log.debug
        this.log = this.debug;
    }

    // In old IE versions, the console isn't present until you first open it.
    // We build realMethod() replacements here that regenerate logging methods
    function enableLoggingWhenConsoleArrives(methodName, level, loggerName) {
        return function() {
            if (typeof console !== undefinedType) {
                replaceLoggingMethods.call(this, level, loggerName);
                this[methodName].apply(this, arguments);
            }
        };
    }

    // By default, we use closely bound real methods wherever possible, and
    // otherwise we wait for a console to appear, and then try again.
    function defaultMethodFactory(methodName, level, loggerName) {
        /*jshint validthis:true */
        return (
            realMethod(methodName) ||
            enableLoggingWhenConsoleArrives.apply(this, arguments)
        );
    }

    function Logger(name, defaultLevel, factory) {
        var self = this;
        var currentLevel;
        var storageKey = "loglevel";
        if (name) {
            storageKey += ":" + name;
        }

        function persistLevelIfPossible(levelNum) {
            var levelName = (logMethods[levelNum] || "silent").toUpperCase();

            if (typeof window === undefinedType) return;

            // Use localStorage if available
            try {
                window.localStorage[storageKey] = levelName;
                return;
            } catch (ignore) {}

            // Use session cookie as fallback
            try {
                window.document.cookie =
                    encodeURIComponent(storageKey) + "=" + levelName + ";";
            } catch (ignore) {}
        }

        function getPersistedLevel() {
            var storedLevel;

            if (typeof window === undefinedType) return;

            try {
                storedLevel = window.localStorage[storageKey];
            } catch (ignore) {}

            // Fallback to cookies if local storage gives us nothing
            if (typeof storedLevel === undefinedType) {
                try {
                    var cookie = window.document.cookie;
                    var location = cookie.indexOf(
                        encodeURIComponent(storageKey) + "="
                    );
                    if (location !== -1) {
                        storedLevel = /^([^;]+)/.exec(
                            cookie.slice(location)
                        )[1];
                    }
                } catch (ignore) {}
            }

            // If the stored level is not valid, treat it as if nothing was stored.
            if (self.levels[storedLevel] === undefined) {
                storedLevel = undefined;
            }

            return storedLevel;
        }

        /*
         *
         * Public logger API - see https://github.com/pimterry/loglevel for details
         *
         */

        self.name = name;

        self.levels = {
            TRACE: 0,
            DEBUG: 1,
            INFO: 2,
            WARN: 3,
            ERROR: 4,
            SILENT: 5
        };

        self.methodFactory = factory || defaultMethodFactory;

        self.getLevel = function() {
            return currentLevel;
        };

        self.setLevel = function(level, persist) {
            if (
                typeof level === "string" &&
                self.levels[level.toUpperCase()] !== undefined
            ) {
                level = self.levels[level.toUpperCase()];
            }
            if (
                typeof level === "number" &&
                level >= 0 &&
                level <= self.levels.SILENT
            ) {
                currentLevel = level;
                if (persist !== false) {
                    // defaults to true
                    persistLevelIfPossible(level);
                }
                replaceLoggingMethods.call(self, level, name);
                if (
                    typeof console === undefinedType &&
                    level < self.levels.SILENT
                ) {
                    return "No console available for logging";
                }
            } else {
                throw "log.setLevel() called with invalid level: " + level;
            }
        };

        self.setDefaultLevel = function(level) {
            if (!getPersistedLevel()) {
                self.setLevel(level, false);
            }
        };

        self.enableAll = function(persist) {
            self.setLevel(self.levels.TRACE, persist);
        };

        self.disableAll = function(persist) {
            self.setLevel(self.levels.SILENT, persist);
        };

        // Initialize with the right level
        var initialLevel = getPersistedLevel();
        if (initialLevel == null) {
            initialLevel = defaultLevel == null ? "WARN" : defaultLevel;
        }
        self.setLevel(initialLevel, false);
    }

    /*
     *
     * Top-level API
     *
     */

    var defaultLogger = new Logger();

    var _loggersByName = {};
    defaultLogger.getLogger = function getLogger(name) {
        if (typeof name !== "string" || name === "") {
            throw new TypeError(
                "You must supply a name when creating a logger."
            );
        }

        var logger = _loggersByName[name];
        if (!logger) {
            logger = _loggersByName[name] = new Logger(
                name,
                defaultLogger.getLevel(),
                defaultLogger.methodFactory
            );
        }
        return logger;
    };

    // Grab the current global log variable in case of overwrite
    var _log = typeof window !== undefinedType ? window.log : undefined;
    defaultLogger.noConflict = function() {
        if (typeof window !== undefinedType && window.log === defaultLogger) {
            window.log = _log;
        }

        return defaultLogger;
    };

    defaultLogger.getLoggers = function getLoggers() {
        return _loggersByName;
    };

    return defaultLogger;
});