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

/*! loglevel - v1.6.8 - https://github.com/pimterry/loglevel - (c) 2020 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) && (typeof window.navigator !== 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;
}));