JavaScript typeof and instanceof – the concept and differences

As a JavaScript developer you must have used the typeof and the instanceof operator sometime in your code to detect the the type of a variable in your program logic. And most often or not you might have been confused with the different results that show up while expecting a desired result. In this post I am gonna show you an easy technique to determine the type and instance of a variable. Let’s see some examples first.

The typeof operator

var str1 = "Hello, how are you?";
typeof str1;
Output: "string"

So, we have a string literal str1, and when we find the typeof str1, it gives “string”. Remember string is a primitive data type in JavaScript. Let’s see another example,

var str2 = new String("I am doing good. Thank you!");
typeof str2;
Output: "object"

Now, we have a String Object and we have created an instance of that Object using the new operator, which is str2. Now if we do a typeof, it gives object. Remember that in JavaScript, other than the primitive data types (and null, undefined, function), everything else is an Object. So as expected you get the type as object.

Now, let’s work with Array’s and try the same thing,

var arr1 = [2,3,4]; //defining an array literal
typeof arr1;
Output: "object"

var arr2 = new Array(4,5,6); //creating an Array Object instance
typeof arr2;
Output: "object"

Firstly, we have an Array literal – arr1 and if we do a typeof it gives “object”. Why? Since Array’s are Objects and they are not part of the primitive data set.
The next example is more obvious, we create an Array Object instance – arr2, and then do a typeof. As expected we get a “object”. So we have seen, if the typeof does not return a primitive data type (which are number, boolean, string), it will return an object. There are some special cases though, look at the bottom of this page to find out.

The instanceof operator

Now let’s take up some examples of the instanceof operator.

var str1 = "Hello, how are you?"; //defining a string literal, same as above
str1 instanceof String;
Output: false

If you see above, we have a string literal and when we do a instanceof to check if the variable is an instance of the String Object, it returns false. Since a string literal belongs to the string primitive data type. Now, consider the example below,

var str2 = new String("I am doing good. Thank you!"); //defining a String object instance
str2 instanceof String;
Output: true

Here, str2 is actually an instance, so the test returns true.
But What’s up with the Array’s though? Let’s see,

var arr1 = [2,3,4]; //defining an array literal
arr1 instanceof Array;
Output: true

var arr2 = new Array(4,5,6); //creating an Array Object instance
arr2 instanceof Array;
Output: true

Both returns true as both arr1 and arr2 are Array objects and they are not part of the primitve data set.

In fact the following test also returns true

arr1 instanceof Object;
Output: true

arr2 instanceof Object;
Output: true

This is because the Array object inherits from the more generic Object object. (Yes, you heard it right Object object!).

Still confused about finding types and instances? Here is the rule to do it easily.

Rule for the test
So, how do you guess if a variable is an instance of its Object or Type. Here is the rule,
1) First check the type of the variable using typeof.
2) Now do the instanceof test.
3) a) If the typeof test returns a primitive type – number, boolean, string. Then instanceof test will return false. Because these are just primitive type data. They are not instances of any Object.
b) But if typeof returns an object, then instanceof test will return true. So you know that you are dealing with objects and instances.
For eg.

var regexp = /d{3}/; //define a regular expression object
typeof regexp; -> "object", not a primitive type
regexp instanceof RegExp; -> true
regexp instanceof Object; -> true

Take another,

var str = "Hello, me again!";
typeof str; ->  "string" , a primitive type
str instanceof String; -> false

Now, you can think of some examples and put the rule to a test.

Special cases
1) typeof null = “object” , since null is a special value in JavaScript which represents emptyness and it does not have any type.
2) typeof undefined = “undefined”, as expected
3) typeof NaN = number, since NaN is actually a special number, although it says Not a Number. Confusing? Just keep in mind.
4) typeof Function = “function”
5) typeof xmlObj = “xml”

1) typeof operator :
2) instanceof operator:

HTML5 SVG Support check using JavaScript – the simple way

My last post talked about how you can detect whether your browser supports HTML5 Canvas or not using simple JavaScript technique. Based on the same principles, this post illustrates how you can detect if your browser supports SVG elements. You may note SVG is Scalable Vector Graphics and HTML5 has native support for SVG tags. The difference with Canvas is that in case of SVG elements, they become a part of the DOM and you can manipulate each SVG elements, whereas in case of Canvas, the entire <canvas></canvas> tag becomes one node in the DOM and anything inside the Canvas <canvas> tag (for eg. a Rectangle) cannot be manipulated individually using standard DOM methods. Browse the web and you will find a hell lot of topics on this.

Alright, now back to the detection code. Just copy/paste this inside your window load / body load or in case of JQuery the $(document).ready() method,

  document.write('Your browser does not support SVG!');

So, if your browser does not support SVG, then it will not process further (in case your application cannot live without SVG). But if you have a back up plan and you can live without SVG in your browser, then you can add an else block along with the if so that you can write your backup logic.

Well, the code is very simple. Inside the if condition I create a SVG element using the standard DOM createElement() method. Now this will create a <svg></svg> object. Now next thing I do is to check if the getAttributeNS() method is defined or not. If you browser does not support <svg> then there is no way it will understand getAttributeNS() and that’s the catch.

HTML5 Canvas Support check using JavaScript – the simple way

This is how you can check for HTML5 Canvas support in your browser by using JavaScript, there’s no need of an external library like Modernizer to do a simple check like this. In fact most of the feature checks can be done natively, that’s what I prefer.

Here is the code,

var canvasEl = document.createElement('canvas'); //create the canvas object
if(!canvasEl.getContext) //if the method is not supported, i.e canvas is not supported
  document.write("HTML5 Canvas Not Supported By Your Browser");

As you can see in the first line I create a HTML5 Canvas element by using createElement() DOM method. As you might know that to work with Canvas, you have to get it’s context object (2d for now), which can be retrieved by the getContext() method. So, if a browser does not support the <canvas> tag or the canvas element, then no way the getContext() method will be defined for it. That’s what I check in the second line – the if condition. Remember that a function/method name is a variable which holds a reference to the function block. So we can check whether it is defined or not, in a normal way like we do for object properties. IE8 doesn’t support HTML5 Canvas. So try this code in IE8 for a quick test.

How can I use it with my code?
Use this code block inside the window load event handler or when the DOM ready event is triggered (using document.onreadystatechange, more). You can modify it as per your need.
Leave behind a comment if you are upset with this attempt.