Checking if data is immutable

Question

What is the quickest and cleanest way to test if a var holds immutable data (i.e. is string, number, boolean, null, undefined)?

E.g. When var test is mutable, the following is legal:

var test = {}; 
//the next 2 lines work because `test` has mutable data
test.someFun = function () { alert(this); };
test.someFun(); 

but when var test is immutable, it's illegal:

var test = "string"; 
//the next 2 lines are invalid, as `test` is a primitive
test.someFun = function () { alert(this); };
test.someFun(); 

Answer 1

Just another short option:

function isPrimitive(value) {
  return Object(value) !== value;
}

How does it works:

If the value is a primitive value that is convertible to object, such as a Number, String or a Boolean, the strict equality operator will return true, since the Object constructor called as a function, will convert the value to a "wrapped object":

typeof 5;         // "number"
typeof Object(5); // "object", a wrapped primitive
5 === Object(5);  // false

If the value is a primitive not convertible to Object, such as null or undefined, the Object constructor will create a new empty object, returning true.

The last case, if value holds a reference to an object, the Object constructor will do nothing, and the strict equality operator will return false.