The only known browser to have actually taken advantage of this is old Internet Explorer see below. In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value.
The type tag for objects was 0. Consequently, null had 0 as type tag, hence the typeof return value "object". Before ECMAScript , typeof was always guaranteed to return a string for any operand it was supplied with. Even with undeclared identifiers, typeof will return 'undefined'. So we fall right at the outset; you simply cannot trust constructor in a codebase that you don't control.
So, you might get unexpected results if the object your testing has a different object set as its prototype. There are ways around this outside the scope of this discussion. There are other uses for the constructor property, some of them interesting, others not so much; for now we will not delve into those uses since it isn't relevant to this discussion. The instanceof operator is a clean way of testing object type as well, but has its own potential issues, just like the constructor property.
But instanceof fails to work for literal values because literals are not Objects. The literals need to be wrapped in an Object in order for instanceof to work, for example. Again, see above; it's quite common for constructor to be utterly and completely wrong and useless.
Using myObjectInstance. For IE9 and above, you can monkey-patch in support :. Updated version from the article in question. This was added 3 months after the article was published, this is the recommended version to use by the article's author Matthew Scharley.
This change was inspired by comments pointing out potential pitfalls in the previous code. It turns out, as this post details , you can use Object. All of these are subject to one potential problem, and that is the question of how the object in question was constructed.
Here are various ways of building objects and the values that the different methods of type checking will return:. While not all permutations are present in this set of examples, hopefully there are enough to provide you with an idea about how messy things might get depending on your needs.
Don't assume anything, if you don't understand exactly what you are after, you may end up with code breaking where you don't expect it to because of a lack of grokking the subtleties. Discussion of the typeof operator may appear to be a glaring omission, but it really isn't useful in helping to identify whether an object is a given type, since it is very simplistic. Understanding where typeof is useful is important, but I don't currently feel that it is terribly relevant to this discussion.
My mind is open to change though. To be precise, I think OP asked for a function that retrieves the constructor name for a particular object. In terms of Javascript, object does not have a type but is a type of and in itself. However, different objects can have different constructors. A blog post linked by Christian Sciberras contains a good example on how to do it. Namely, by extending the Object prototype:. It turns out, as this post details, you can use Object.
Here is a solution that I have come up with that solves the shortcomings of instanceof. It can check an object's types from cross-windows and cross-frames and doesn't have problems with primitive types. The real trick to how it works is that it checks if the object is from the same window and if not gets the object's window. The type argument can also be a callback function which returns a constructor.
The callback function will receive one parameter which is the window of the provided object. I was actually looking for a similar thing and came across this question.
Here is how I get types: jsfiddle. The kind function from Agave. JS will return:. It works on all JS objects and primitives, regardless of how they were created , and doesn't have any surprises. You can use the instanceof operator to see if an object is an instance of another, but since there are no classes, you can't get a class name. Here is an implementation based on the accepted answer :.
You can use the "instanceof" operator to determine if an object is an instance of a certain class or not. If you do not know the name of an object's type, you can use its constructor property. This object is called a prototype. An object can obtain properties and methods from the constructor function from which the object is created; from its prototype, through inheritance; and from any properties and methods added separately to the object after it has been constructed remember that Functions are just data that can be freely assigned to variables, as and when required.
Two objects made from the same constructor function can have different properties and methods from each other, since extra properties and methods can be added to each object after they have been created. Associated with inheritance is the separate concept of polymorphism , the ability to call a method on an object irrespective of the constructor function used to create the object, and expect the appropriate method to be called for the given object.
For example, there can be two constructor functions, Square and Circle , which create objects representing squares and circles, respectively. The constructor can create these objects so that they each have an area method that returns the area of the object.
Of course, the method will operate differently for square and circle objects, but any variable holding either a square or a circle object can have the area method called on it, and polymorphism will ensure that the correct area method is called for the appropriate object.
Although it can initially seem rather pedantic, the following naming convention is used throughout this unit, and is widely used in various programming standards. Using a standard convention helps a programmer to understand the code that they are reading, and you are advised to adopt this convention in your own programming.
All words of a constructor are spelt with an initial capital letter. Examples include:. The first word in an instance object name is spelt with all lower case letters; all subsequent words making up the object name are spelt with an initial capital, as per usual. The first word of a method name is spelt in lower case; all subsequent words making up the object name are spelt with initial capitals. The first word of a property is spelt in lower case; all subsequent words making up the object name are spelt with initial capitals.
You can find a discussion of this Exercise at the end of the unit. Objects are created in a number of ways, although the most common is by the use of the new operator. This is a unary, prefix operator, which means it is placed before a single operand. The new operator should be followed by a constructor function. Sufficient free memory is located in which to create the new object.
The new operator returns a reference to the location in memory where the new object has been created. In the first of the three examples above new Object ; , no variable is assigned the result of the object creation, therefore there is no way for the JavaScript programmer to refer to this object later in the programme.
In almost all cases, a variable will be assigned the reference to the location of the newly created object. In the second example above the reference variable accountNumbers is assigned the reference to the location of the newly created Array object. Variables do not store objects, they store a reference to the object located elsewhere in memory. Although the following way of creating String objects is perfectly acceptable JavaScript:.
The same result could be achieved with the following statement:. String objects can be created in two different ways because the creation of String objects is such a common feature of programming that the developers of the JavaScript language provided this second, simpler syntax especially for Strings.
Just as the creation of Strings is so common the JavaScript developers provided a special syntax, the same has been done for make the creation of Function objects easier. As you will know from earlier units a function object can be created in the following way:. Functions are themselves objects of the type Function , and can be created in the same way as objects using the Function constructor:. In most cases the former way of creating Function objects is more straightforward.
Objects have both data properties and methods function properties. Often a property of one object is an object in its own right or perhaps another kind of collection, such as an array.
An example is the forms[ ] array property of the document objects — this property contains details i. Variables data that "belong" to objects are called properties.
In earlier units you have seen examples of properties, such as an array's length property:. In the example above the length property of the myArray object is accessed by the dot notation:.
Methods are object properties containing functions. In earlier units you have seen examples of methods, for example the reverse method of an array:. In the example above the reverse method of the myArray object is invoked by the dot notation:. As can be seen, the only difference from data properties is when methods are invoked with the use of the parentheses operator — this is a special operator expecting a variable holding a Function object to the left, and optional operands the function arguments between the parentheses.
It is important to realise that methods are properties , but ones which can also be invoked since they are properties that hold Function objects. What is sometimes known as dot notation is used to indicate which properties an object owns. Ownership of properties can exist to any number of levels deep, and the dot notation can be used at every level. The use of the full stop dot can be thought of as meaning "the thing on the right belongs to the object named collection on the left".
The general form for properties and methods to be invoked is as follows:. In each of the above examples, the dot syntax is used to refer to a named piece of data property of an object. So we can refer to the length property belonging to the Array object myArray, and the src property of the Image object catpic. This line refers to the value property of the age property itself an object of the form object at index 0 of the forms[] array object of the document object.
Code using such a reference to the value of a form's text input box is as follows:. When a JavaScript programme is being executed. The global object exists until the programme terminates. All variables of a JavaScript programme are properties of this global object.
In the same way that all variables of a JavaScript programme are in fact properties of the global object, likewise all the functions of a JavaScript programme are methods of the global object. Since methods are just properties that hold Function objects, functions, being methods of the global object, are also stored in properties of the global object. Although you have been programming with objects for some weeks in JavaScript, you may not have been aware of some of the objects you have been working with.
Any variable can be emptied, by setting the value to undefined. The type will also be undefined. We just launched W3Schools videos. Get certified by completing a course today! If you want to report an error, or if you want to make a suggestion, do not hesitate to send us an e-mail:. When adding a number and a string, JavaScript will treat the number as a string. Report Error.
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