Callbacks

A callback function is a function passed into another function as an argument.
This can then be invoked inside the outer function to complete some kind of routine or action.

Synchronous callback

function greeting(name) { 
  alert('Hello ' + name); 
} 

function processUserInput(callback) { 
  var name = prompt('Please enter your name.'); 
  callback(name); 
} 

processUserInput(greeting); 

function ask(question, yes, no) { 
  if (confirm(question)) yes() 
  else no(); 
} 

ask( 
  "Do you agree?", 
  function() { alert("You agreed."); }, 
  function() { alert("You canceled the execution."); } 
); 

Asynchronous Callback

function asyncMethod(message, callback_function) { 
  setTimeout( function() { 
    console.log(message); 
    callback_function(); 
   }, 400) 
} 

asyncMethod('Connect to Database', function() { 
} 

Nested Callbacks

asyncMethod('Connect to Database', function() { 
  asyncMethod('Verify User Details', function() { 
    asyncMethod('Check Permissions', function() { 
      asyncMethod('Update User', function() {} ) 
    }) 
  }) 
}) 

Best Practices

Arguments should be (err, results)
Always 'return' when you want to exit