http-proxy-middleware/lib/logger.js

var util = require('util');
var _ = require('lodash');

var loggerInstance;

var defaultProvider = {
    log: console.log,
    debug: console.log,    // use .log(); since console does not have .debug()
    info: console.info,
    warn: console.warn,
    error: console.error
};

// log level 'weight'
var LEVELS = {
    debug: 10,
    info: 20,
    warn: 30,
    error: 50,
    silent: 80
};

module.exports = {
    // singleton
    getInstance: function() {
        if (!loggerInstance) {
            loggerInstance = new Logger();
        }

        return loggerInstance;
    },
    getArrow: getArrow
};

function Logger() {
    var logLevel;
    var provider;

    var api = {
        log: log,
        debug: debug,
        info: info,
        warn: warn,
        error: error,
        setLevel: function(v) {
            if (isValidLevel(v)) {
                logLevel = v;
            }
        },
        setProvider: function(fn) {
            if (fn && isValidProvider(fn)) {
                provider = fn(defaultProvider);
            }
        }
    };

    init();

    return api;

    function init() {
        api.setLevel('info');
        api.setProvider(function() {
            return defaultProvider;
        });
    }

    // log will log messages, regardless of logLevels
    function log() {
        provider.log(_interpolate.apply(null, arguments));
    }

    function debug() {
        if (_showLevel('debug')) {
            provider.debug(_interpolate.apply(null, arguments));
        }
    }

    function info() {
        if (_showLevel('info')) {
            provider.info(_interpolate.apply(null, arguments));
        }
    }

    function warn() {
        if (_showLevel('warn')) {
            provider.warn(_interpolate.apply(null, arguments));
        }
    }

    function error() {
        if (_showLevel('error')) {
            provider.error(_interpolate.apply(null, arguments));
        }
    }

    /**
     * Decide to log or not to log, based on the log levels 'weight'
     * @param  {String}  showLevel [debug, info, warn, error, silent]
     * @return {Boolean}
     */
    function _showLevel(showLevel) {
        var result = false;
        var currentLogLevel = LEVELS[logLevel];

        if (currentLogLevel && (currentLogLevel <= LEVELS[showLevel])) {
            result = true;
        }

        return result;
    }

    // make sure logged messages and its data are return interpolated
    // make it possible for additional log data, such date/time or custom prefix.
    function _interpolate() {
        var fn = _.spread(util.format);
        var result = fn(_.slice(arguments));

        return result;
    }

    function isValidProvider(fnProvider) {
        var result = true;

        if (fnProvider && !_.isFunction(fnProvider)) {
            throw new Error('[HPM] Log provider config error. Expecting a function.');
        }

        return result;
    }

    function isValidLevel(levelName) {
        var validLevels = _.keys(LEVELS);
        var isValid = _.includes(validLevels, levelName);

        if (!isValid) {
            throw new Error('[HPM] Log level error. Invalid logLevel.');
        }

        return isValid;
    }
}

/**
 * -> normal proxy
 * => proxyTable
 * ~> pathRewrite
 * ≈> proxyTable + pathRewrite
 */
function getArrow(originalPath, newPath, originalTarget, newTarget) {
    var arrow = ['>'];
    var isNewTarget = (originalTarget !== newTarget); // proxyTable
    var isNewPath = (originalPath !== newPath); // pathRewrite

    if (isNewPath && !isNewTarget) {arrow.unshift('~');} else if (!isNewPath && isNewTarget) {arrow.unshift('=');} else if (isNewPath && isNewTarget) {arrow.unshift('≈');} else {arrow.unshift('-');}

    return arrow.join('');
}