On many sites, I have read that for example these two are different:
var s1 = String('s1'); // typeof s1 == 'string'
var s2 = new String('s2'); // typeof s2 == 'object'
The same applies on booleans, numbers, etc.
However, I did not find any useful information on WHY these two constructs produce different kind of objects. Is there any specific reason for that? Are there any cases when I would leverage both approaches?
var s1 = String('s1');
Calls the function String which returns a string primitive.
var s2 = new String('s2');
Calls String as a constructor which returns an object, that is an instance of String.
String knows when it's called in a non-constructor context. The creator of the language has deliberately decided not to return an object in that case.
The only useful purpose I can imagine for the first example is type conversion, like String(4), which creates '4'. But there are other ways to achieve this.
There does not seem to be a good use case for the second example. Directly creating a wrapper object can even be harmful when the code expects a primitive. E.g.:
var s2 = new String('s2');
doSomethingUseful(s2);
function doSomethingUseful(aParameter) {
if (typeof aParameter === 'string') { //s2 isn't
new?
new. (Though using Number() in a similar way is a more likely scenario than using String().)Here's a mini FAQ for you.
String and Number objects?Primitive values, like "foobar" or 123 cannot have properties and methods, so there must be objects that actually hold these.
String and Number objects are used?They're used implicitly by the engine itself. Every time you assess a property of a primitive, the engine creates a new implicit object just for this (this is called "autoboxing") . For example x = "foobar".length is turned into this:
temp = new String("foobar")
x = temp.length
delete temp
String and Number objects in my own code?You can, but hardly need. Due to automatic boxing, you can call all objects' props and methods directly on primitive values. Also, primitives and boxed objects behave differently under some circumstances, e.g. if(x)... vs. if(new String(x))....
String() and Number() functions?These are constructors for String and Number objects. Additionally, when called directly, without new, they perform type conversion, i.e. return a new primitive (not object) of their respective type. In pseudocode, the logic behind String is like this:
function String(primitiveValue) {
primitiveString = convert_to_string(primitiveValue)
if called with new
return new StringObject(primitiveString)
else
return primitiveString
Reference: http://es5.github.io/#x15.5.1
String() and Number() useful?Yes! Every time a type casting is required you want to use these. Some people prefer hacks, like +x instead of Number(x) or x+"" instead of String(x), but explicit casting looks cleaner.
typeof new String() is object and not string?Everything created by the means of new is an object, no matter which specific constructor is used.
Well String(), Function() or Boolean() are all objects of Function reference type.
Before discussing the question u should know there are two data types in javascript.
1)Primitive -for simple string,number,undefined,null,object values etc. cantaining variables.They can be differentiated by typeof operator.For object this will only tell if the given variable contains reference type of data and not what kind of reference type.
2)reference types-they are just like classes in other oop languages.They are the types of objects that can be created by javascript.Some Examples are-function,Date,regex,primitive Wrapper Type etc.They can be differentiated by instanceof operator.
Primitive wrapper Types-The are actually the objects that contain the primitive values but they behave like objects passing their values by reference.They are created by calling the constructor functions with new operator.Ex- String-They are objects containing String kind of Values.Syntax for creating these objects
var str=new String("s2")
here str behaves as an object.
typeof(str)=="object"
Whereas just calling String() is just any calling any ordinary function whose only purpose is to convert any other primitive value into string.
var s1 = String('s1'); // typeof s1 == 'string'
I hope I was clear.If you have any doubts then ask them
newoperator withString(),Number()orBoolean().