Readings: Callbacks

• Watch the Shred Talk: Callbacks
• Review the Demo Code
• Complete the Assignment

Javascript Callback Pattern

Node.js made the decision to have all asynchronous events be handled using error first callbacks. The main advantage of this is that all asynchronous methods have a consistent interface. This means that when you are working with asynchronous Node.js code, you can always assume how the callback is going to be formatted, making your life as a developer much easier!

Having a consistent callback interface has also made it possible for libraries to be written that assist Javascript developers in handling complex async code!

A callback says, “Hey, Javascript, you go ahead do some work. I don’t care how long it takes, and I’m going to go ahead and keep on working … but when you’re all done, here’s a function I want you to run.”

That function is the callback, and it operates on a nicely standardized signature.

Defining an Error First Callback

• a callback is simply a function that is passed as an argument to another function
• “error first” callbacks have the function signature (err, result) => {}
  • the first parameter is reserved for an error
    • the value will be null or undefined if there is no error
  • the second callback is reserved for any successful response data
    • the value will be null or undefined if there is no data
    • not every Node.js method passes data into the callback
    • in methods that do not resolve data, success is defined as a lack of an error

Additional Resources

• Video: what the heck is the event loop anyway
• error first callbacks
• fs module docs