Promises

Added in ES6
Promises are similar to event listeners with a few important differences.
Lets you separate the parameters from the function code.
link - promisesjs.org
A promise can only suceed or fail once

Needed to add a Promises reference a few years ago ??

Nested Callbacks

Add the 'then'

function asyncMethod(message) { 
  return new Promise( function (succeed, fail) { 
    setTimeOut( function() { 
      console.log(message); 
      succeed(); 
     }, 400) 
  }); 
} 

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

Step 2 - Remove the anonymous functions

function asyncMethod(message) { 
  return new Promise( function (succeed, fail) { 
    setTimeOut( function() { 
      console.log(message); 
      succeed(); 
     }, 400) 
  }); 
} 

function fn_verifyUserDetails() { 
  asyncMethod('Verify User Details') 
    .then( fn_checkPermissions ) 
} 
function fn_checkPermissions() { 
  asyncMethod('Check Permissions') 
    .then( fn_updateUser ) 
} 
function fn_updateUser() { 
  asyncMethod('Update User') 
    .then( function () {} ) 
} 
asyncMethod('Connect to Database') 
  .then( fn_verifyUserDetails ) 

Step 3 - Chain together

function asyncMethod(message) { 
  return new Promise( function (succeed, fail) { 
    setTimeOut( function() { 
      console.log(message); 
      suceed(); 
     }, 400) 
  }); 
} 

function fn_verifyUserDetails() { 
  return asyncMethod('Verify User Details') 
} 
function fn_checkPermissions() { 
  return asyncMethod('Check Permissions') 
} 
function fn_updateUser() { 
  return asyncMethod('Update User') 
} 
asyncMethod('Connect to Database') 
  .then( fn_verifyUserDetails ) 
  .then( fn_checkPermissons ) 
  .then( fn_updateUser ) 
  .then( function () {} )