asynchronous_handler.js

exports.handler = async function(event, context) {   

  // Your code goes here ...
  
  let success = function() {
    // Do stuff ...
  };
  
  // Exit the handler by invoking the callback.
  // The first argument is an error object (null indicates success).
  // The second argument is the result. It must be JSON.stringify compatible.
  if (success) {
    callback(null, "Success :)");
  } else {
    callback("We tried :("); 
  }
}

synchronous_handler.js

exports.handler = function(event, context, callback) {   

  // Your code goes here ...
  
  let success = function() {
    // Do stuff ...
  };
  
  // Exit the handler by invoking the callback.
  // The first argument is an error object (null indicates success).
  // The second argument is the result. It must be JSON.stringify compatible.
  if (success) {
    callback(null, "Success :)");
  } else {
    callback("We tried :("); 
  }
}