FUNCTIONS

First, strict mode requires that named function arguments be unique. Consider the following:

// Duplicate named arguments
// Non-strict mode: No error, only second argument works
// Strict mode: Throws a SyntaxError
function sum (num, num){
 // do something
}

In nonstrict mode, this function declaration doesn't throw an error. You'll be able to access the second num argument only by name while the first is accessible only through arguments.

The arguments object also has a slight behavior change in strict mode. In nonstrict mode, changes to a named argument are also reflected in the arguments object, whereas strict mode ensures that each are completely separate. For example:

// Change to named argument value
// Non-strict mode: Change is reflected in arguments
// Strict mode: Change is not reflected in arguments
function showValue(value){
 value = "Foo";
 alert(value);        // "Foo"
 alert(arguments[0]); // Non-strict mode: "Foo"
...