A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine A good example is the callback functions executed inside a .then() block chained onto the end of a promise after that promise fulfills or rejects What Are Callbacks? A Callback is simply a function passed as an argument to another function which will then use it (call it back). Code that looks like the above has been named Callback Hell. Async JS doesn't need to look like this, but it's easy to carelessly write a few functions and realize.. What is a callback, anyway? Back in my early days of noobhood, I started taking a look at Node.js and quickly got overwhelmed with all of these strange things Wikipedia actually does a fairly sensical job of defining callbacks. In other words, a callback is a function that gets passed into another function.. Node.js - Callbacks Concept - Callback is an asynchronous equivalent for a function. A callback function is called at the completion of a given task. Once file I/O is complete, it will call the callback function while passing the callback function, the content of the file as a parameter What is a Callback Function? The above-linked Wikipedia article defines it nicely The callback function itself is defined in the third argument passed to the function call. Yes, you're right, JS lines will load in order of appearance. However, as I understand things, the new lines that run don't wait for..