The 'interesting' JavaScript Types

20-11-2014

typeof primitive types

Here lists the typeof results of primitive types:

var undefinedVariable;
var nullValue = null;
var anObject = new Object();
var func = function () { };

typeof ("string"); //result in "string"
typeof (1024); // number
typeof (true); // boolean
typeof (undefinedVariable); // undefined
typeof (anObject ); // object
typeof (undeclaredValue); // undefined
typeof (nullValue); // object
typeof (null) // object
typeof (func) // function
typeof (Object) // function

Things need to be highlighted are:

  • Both undefined and undeclared variable are of type undefined
  • Variable is null when assigned with null otherwise it will be undefined
  • Object is a function
  • null is an object, I guess it means a null object.

Object is a function rather than an object

Object is actually a constructor function to create objects (that why typeof(Object) result in function), just like the Person function shown below.

var alice= new Object();
alice.name = "Alice";
//or alice = { name : "Alice" };, the result will be the same.
console.log(alice.constructor.name);// Object;
console.log(typeof alice);// object
console.log(typeof Object);// fucntion

function Person(name){
    this.name = name;
}

var bob = new Person("Bob");
console.log(typeof bob); // object
console.log(typeof Person); // function
console.log(bob.constructor.name); // Person

Similarly, the upper case String, Number, Boolean are functions as well as Object:

typeof(String);  // function
typeof(Number);  // function
typeof(Boolean); // function

typeof(string);  //string is not defined
typeof(boolean); // boolean is not defined

var s = new String("test");
typeof(s); // object
s.constructor.name;; // String
"test".constructor.name; // String

##Other functions##

Array, Date, RegExp are functions as well.

typeof(Array);// function
typeof(new Array());// object
typeof(Date); // function
typeof(new Date()); // object
typeof(RegExp); // function
typeof(new RegExp());// object

Now, how can we determine my object is of a specific function? The answer is instanceof

new Array() instanceof Array;// true
[] instanceof Array; // true
[].constructor.name; // Array

##function## functions are first-class objects, they are objects and can be manipulated and passed around just like any other object. Specifically, they are Function objects, just like the relationship between Object and object.

typeof(Function); // function
typeof(function(){}); // function
var x = function(){};
x.constructor.name; // Function
x instanceof Function; // true
function y(){}
typeof(y); //function
y.constructor.name; // Function
y instanceof Function; // true

We can use Function to define a function as well:

// equivalent to:
// var sayHello = function(name){ alert("Hello  " + name  ); }
var sayHello = new Function('name','alert("Hello " + name  );');
sayHello('Alice');// result in 'Hello Alice'
typeof(sayHello); // function
sayHello instanceof Function;// true
sayHello.constructor.name; // Function

##So##

  • Function is a function to construct functions
  • Object is a function to construct objects, em..
  • Seems String suppose to be a function to construct strings, but actually objects, however the new String("") is instanceof String;
  • ”” instanceof String;// false as “” is primitive string, not a object of String
  • typeof(“”) // string
  • typeof(new String(“”)); //object
  • So seems using the Uppercase Functions(e.g. String) is a bad idea…
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