Programming JavaScript Applications

Chapter 7. Logging

Logging is a critical piece of application infrastructure, particularly in a modern JavaScript application. A typical JavaScript application will involve events in a browser, events on the server, events related to third-party API communication, and a real need for deep profiling and analysis of application performance characteristics. Logging serves every aspect of the application business, including:

• Debugging

• Server operations

• Security/intrusion detection

• Auditing

• Business analytics

• Marketing

Debugging

For debugging, it’s critical to log all application errors and warnings. The log messages that serve other functions will also come in handy while you’re debugging, because they can supply context when something goes wrong.

During development, it’s often useful to log debug messages to investigate specific issues. In those scenarios, it’s useful to log:

• The value of a related variable

• If a particular function was called

• The return value for a function call

• API responses

• The sequence of particular events

Sometimes it’s useful to know which functions called which at a particular spot in your codebase. It’s possible to log a stack trace with console.trace():

  var foo = function foo() {
      console.trace();
    },
    bar = function bar() {
      foo();
    };

  bar();

  /*
  Trace
      at foo (repl:2:9)
      at bar (repl:5:1)
      at repl:1:1
      at REPLServer.self.eval (repl.js:110:21)
      at repl.js:249:20
      at REPLServer.self.eval (repl.js:122:7)
      at Interface.<anonymous> (repl.js:239:12)
 at Interface.EventEmitter.emit ...