AuraHandledException

From Apex class:

 @AuraEnabled

    public static List<Account> fetchAccount() {

        throw new AuraHandledException('User-defined error');

    }

From client-side controller:

doInit: function(component, event, helper) {

        var action = component.get('c.fetchAccount');

        action.setParams({ firstName : cmp.get("v.firstName") });

        action.setCallback(this, function(response) {

            var state = response.getState();

            if (state === "SUCCESS") {

                component.set('v.Accounts', response.getReturnValue());

            }

            else if (component.isValid() && state === "ERROR") {

                console.log("Error Message: ", response.getError()[0].message);

            }

        });

        $A.enqueueAction(action);

    }

How to Select All Fields with SOQL in Apex

1. Query All Fields for a Known Record ID:

ID recordId = '5001a00000CgCE2';

DescribeSObjectResult describeResult = recordId.getSObjectType().getDescribe();

List<String> fieldNames = new List<String>( describeResult.fields.getMap().keySet() );

String query =
  ' SELECT ' +
      String.join( fieldNames, ',' ) +
  ' FROM ' +
      describeResult.getName() +
  ' WHERE ' +
      ' id = :recordId ' +
  ' LIMIT 1 '
;

// return generic list of sobjects or typecast to expected type
List<SObject> records = Database.query( query );

System.debug( records );

2. Query All Fields for All Records

The same concept as above but we don’t have a specific record ID, so we need to determine the sobjectype by explicitly specifying the object we’ll query on.

// without an ID, simply specify the object to then derive the sobject type
DescribeSObjectResult describeResult = Account.getSObjectType().getDescribe();

List<String> fieldNames = new List<String>( describeResult.fields.getMap().keySet() );

String query =
  ' SELECT ' +
      String.join( fieldNames, ',' ) +
  ' FROM ' +
      describeResult.getName()
;

// return generic list of sobjects or typecast to expected type
List<SObject> records = Database.query( query );

System.debug( records );

JavaScript in Strict Mode

JavaScript code that runs in a browser can access a user's private information. To protect this information, LockerService requires you to use ECMAScript (ES5) strict mode, and enforces it in your code. This mode makes it easier to write secure JavaScript. A few code tweaks to comply with ES5 can substantially reduce the need for checking data at runtime.

Here’s a simple example of how strict mode prevents JavaScript from traversing the call-stack and modifying other functions. Without strict mode, the following function would have returned the name of the function that called primaryFunction().

JavaScript code:
function primaryFunction() {
  'use strict'
  console.log("The function calling the current method is: “+ primaryFunction.caller.name);
}
function callerFunction() {
  return primaryFunction(); 
}
callerFunction();

Output:
Uncaught TypeError: 'caller' and 'arguments' are restricted function properties and cannot be accessed in this context.
at primaryFunction (<anonymous>:3:72) at callerFunction (<anonymous>:6:10) at <anonymous>:8:1

Strict mode restricts access to several other entities. When it is enforced, you can no longer reference the window object through the JavaScript variable this, for example. These restrictions eliminate a lot of potential security vulnerabilities.