Asynchronous code with callbacks in JavaScript

Let's have some asynchronous function:

javascriptfunction make() {
  setTimeout(function() {
    console.log('1');
  }, 3000);
}

Let's use this function like this:

javascriptmake();
console.log('2'); // it will be executed first

Let's say we want to make the second output to the console execute after an asynchronous operation within the function is completed. One approach used for this is to use callback: let's wrap the waiting code in an anonymous function and pass as a parameter to the make function:

javascriptmake(function() {
  console.log('2');
});

Of course, this alone will not solve our problem. For now, we've just made the following agreement: if you want to execute a code after make fires, pass that code as a callback to the make call.

Let's fix the code of the make function so that it starts working in accordance with our agreement:

javascriptfunction make(callback) {
  setTimeout(function() {
    console.log('1'); // some asynchronous operation, maybe more than one
    callback();       // then our callback
  }, 3000);
}

javascriptfunction make(callback) {
  setTimeout(function() {
    console.log('1');
    callback();
  }, 3000);
}

make(function() {
  console.log('2');
  console.log('3');
});