VARIABLES

How and when variables get created is different in strict mode. The first change disallows accidental creation of global variables. In nonstrict mode, the following creates a global variable:

// Variable is not declared
// Non-strict mode: creates a global
// Strict mode: Throws a ReferenceError
message = "Hello world!";

Even though message isn't preceded by the let keyword and isn't explicitly defined as a property of the global object, it is still automatically created as a global. In strict mode, assigning a value to an undeclared variable throws a ReferenceError when the code is executed.

A related change is the inability to call delete on a variable. Nonstrict mode allows this and may silently fail (returning false). In strict mode, an attempt to delete a variable causes an error:

// Deleting a variable
// Non-strict mode: Fails silently
// Strict mode: Throws a ReferenceError
let color = "red";
delete color;

Strict mode also imposes restrictions...