Why is 4 not an instance of Number?

Question

value instanceof Constructor is the same as Constructor.prototype.isPrototypeOf(value) and both check the [[Prototype]]-chain of value for occurences of a specific object.

Strings and numbers are primitive values, not objects and therefore don't have a [[Prototype]], so it'll only work if you wrap them in regular objects (called 'boxing' in Java).

Also, as you noticed, String(value) and new String(value) do different things: If you call the constructor functions of the built-in types without using the new operator, they try to convert ('cast') the argument to the specific type. If you use the new operator, they create a wrapper object.

new String(value) is roughly equivalent to Object(String(value)), which behaves the same way as new Object(String(value)).

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Some more on types in JavaScript: ECMA-262 defines the following primitive types: Undefined, Null, Boolean, Number, and String. Additionally, there is the type Object for things which have properties.

For example, functions are of type Object (they just have a special property called [[Call]]), and null is a primitive value of type Null. This means that the result of the typeof operator doesn't really return the type of a value...

Aditionally, JavaScript objects have another property called [[Class]]. You can get it via Object.prototype.toString.call(value) (this will return '[objectClassname]'). Arrays and functions are of the type Object, but their classes are Array and Function.

The test for an object's class given above works when instanceof fails (e.g. when objects are passed between window/frame boundaries and don't share the same prototypes).

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Also, you might want to check out this improved version of typeof:

function typeOf(value) {
    var type = typeof value;

    switch(type) {
        case 'object':
        return value === null ? 'null' : Object.prototype.toString.call(value).
            match(/^\[object (.*)\]$/)[1]

        case 'function':
        return 'Function';

        default:
        return type;
    }
}

For primitives, it will return their type in lower case, for objects, it will return their class in title case.

Examples:

• For primitives of type Number (eg 5), it will return 'number', for wrapper objects of class Number (eg new Number(5)), it will return 'Number';

• For functions, it will return 'Function'.

If you don't want to discern between primitive values and wrapper objects (for whatever, probably bad reason), use typeOf(...).toLowerCase().

Known bugs are some built-in functions in IE, which are considered 'Object' and a return value of 'unknown' when used with some COM+ objects.

You may try to evaluate:

>>> typeof("a")
"string"
>>> typeof(new String("a"))
"object"
>>> typeof(4)
"number"
>>> typeof(new Number(4))
"object"

As stated in Christoph's answer, string and number literals are not the same as String and Number objects. If you use any of the String or Number methods on the literal, say

'a string literal'.length

The literal is temporarily converted to an object, the method is invoked and the object is discarded.
Literals have some distinct advantages over objects.

//false, two different objects with the same value
alert( new String('string') == new String('string') ); 

//true, identical literals
alert( 'string' == 'string' );

Always use literals to avoid unexpected behaviour!
You can use Number() and String() to typecast if you need to:

//true
alert( Number('5') === 5 )

//false
alert( '5' === 5 )

In the case of primitive numbers, the isNaN method could also help you.

This is a nuance of Javascript which I've found catches some out. The instanceof of operator will always result in false if the LHS is not an object type.

Note that new String('Hello World') does not result in a string type but is an object. The new operator always results in an object. I see this sort of thing:

function fnX(value)
{
     if (typeof value == 'string')
     {
          //Do stuff
     }
}
fnX(new String('Hello World'));

The expectation is that "Do Stuff" will happen but it doesn't because the typeof the value is object.