Locker Service automatically enforces ES5 strict mode for the JavaScript. If you write JavaScript that is not valid in strict mode, you will see unexpected errors. Let's take a look at some of the best practices that one must follow for the JavaScript code in controllers and helper files:

• Using a variable without declaring it is not allowed. Suppose that you declare a variable as follows:

a = component.set("v.name",'Test'); 

 This is invalid; instead, use the var keyword. The correct syntax is as follows:

var a = component.set("v.name",'Test');

• Using an object without declaring it is also not allowed:

x = {x:4, y:20};      // This will cause an error

   The fix for the preceding is as follows:

var x = {x:4, y:20};      // This will cause an error

• Deleting a variable (or an object) is not allowed. The following is not allowed:

var x = 5;
delete x;

• Deleting a function (or an object) is also not allowed:

function x(p1, p2) {};
delete x;

• Duplicating a parameter name is not allowed:

function x(p1, p1) {};

• Writing to a read-only property is not allowed:

var obj = {};
Object.defineProperty(obj, “x”, {value:0, writable:false});
obj.x = 3;

• Writing to a get-only property is not allowed:

var obj = {get x() {return 0} };
obj.x = 3;

• Writing to a get-only property is not allowed:

var obj = {get x() {return 0} };
obj.x = 3;

The following future reserved keywords are not allowed in strict mode:

• implements
• interface
• let
• package
• private
• protected
• public
• static
• Yield