Callbacks are first-class functions. This means they have first-class characteristics, like being able to be passed to other functions. There was a time when callbacks were used to handle async operations, but we needed something better because of a few shortcomings (like problems with nested callbacks).

Here's a simple example of a callback:

function asyncFunc(cb) {
  let result = ""
  // After some time the result of an async opertion comes back and is put in the "result" variable.  We'll use a string for this example.   
  result = "hello"
  cb(result)
}

function callback(str) {
  console.log(str)
}

asyncFunc(callback)
console.log("goodbye")

// goodbye
// hello

"goodbye" appears before "hello" because the async operation in asyncFunc() is non-blocking, meaning that it is set aside until it finishes but in the meantime we go ahead and call the next function.

Challenge

• Fix anotherFunc() so that calls to it will change the doc variable to bye.
• Keep the setTimeout to 100ms and do not change the callback function or the doc variable.

Notes

Check the Resources tab for more info on callbacks.