fix: assign log levels to un bound functions

lib/winston/create-logger.js

@@ -45,7 +45,8 @@ class DerivedLogger extends Logger {
 
       // Define prototype methods for each log level
       // e.g. logger.log('info', msg) <––> logger.info(msg) & logger.isInfoEnabled()
-      this[level] = (...args) => {
+      // this is not an arrow function so it'll always be called on the instance instead of a fixed place in the prototype chain.
+      this[level] = function (...args) {
         // Optimize the hot-path which is the single object.
         if (args.length === 1) {
           const [msg] = args;

test/logger.test.js

@@ -66,6 +66,30 @@ describe('Logger', function () {
     })
   });
 
+  it('new Logger({ levels }) custom methods are not bound to instance', function () {
+    var logger = winston.createLogger({
+      level: 'error',
+      exitOnError: false,
+      transports: []
+    });
+
+    let logs = [];
+
+    let extendedLogger = Object.create(logger,{
+      write:{
+        value:function(...args){
+          logs.push(args);
+        }
+      }
+    });
+
+    extendedLogger.log({test:1});
+    extendedLogger.warn({test:2});
+
+    assume(logs[0]||[]).is.eql([{test:1}]);
+    assume(logs[1]||[]).is.eql([{message:{test:2},level:'warn'}]);
+  });
+
   it('.add({ invalid Transport })', function () {
     var logger = winston.createLogger();
     assume(function () {