JavaScript Explained

What is a CDN and How to load jQuery from CDN with Fallback?

What is a CDN ?

CDN is an acronym for Content Delivery Network.
From wiki "A content delivery network or content distribution network (CDN) is a large distributed system of servers deployed in multiple data centers across the Internet. The goal of a CDN is to serve content to end-users with high availability and high performance."

As a developer we mostly use CDNs for loading some JavaScript releated JS plugins and frame works like jQuery, Dojo, AngulaJS etc.. However CDN Not only used for downloading text/Script files but Its major use now a days is downloading objects like media files, software, documents etc.., applications like e-commerce, portals And also for live streaming media.

Which are the popular jQuery CDN ?

In this post we want to concentrate on jQuery related CDNs only so I ll be listing only jQuery related CDN sources mainly. You can use them in your application.

CDNs can offer a performance benefit by hosting jQuery on servers spread across the globe. This also offers an advantage that if the visitor to your webpage has already downloaded a copy of jQuery from the same CDN, it won't have to be re-downloaded.
Popular CDNs and their URLs

jQuery's CDN provided by MaxCDN
Use
Google CDN
Use
Microsoft CDN
Use
CDNJS CDN
Use
jsDelivr CDN
Use

How to load jQuery locally when CDN fails?

There May be Situations where JavaScript file from CDN May not be loaded. There can be any reason for it as i mentioned it in my blog Here How to Check If JQuery Library Loaded?
WHat to do in this situation? Its simple just check If the jQuery library is loaded or not using This method and add the local JS file in Else condition as shown below.

That is it. Isn't it that simple???

How to Check If JQuery Plugin or Library is loaded?

There may be some situations where we need to know whether a JQuery plugin/library is loaded or not loaded.
For Example if we are using any CDN to load JQuery in our application and some times CDN may be blocked by the user or CDN Server is not reachable/down. So Its always better to check if jQury plugin is loaded or not once before using it.

To check if jQuery library is loaded?

to check if jQuery library and plugin is loaded?

to check if jQuery library

if (typeof jQuery != 'undefined') {
     alert("jQuery library is loaded!");
 }

Or We can Check like below

if (jQuery) { 
   alert("jQuery library is loaded!");
}

Using Above methods we can Check If JQuery library is loaded r not. To Check If any particular jQuery plugin is loaded or not, we can use below method.

To check if jQuery Plug-in is loaded?

if(jQuery().pluginMethod) {
    alert("jQuery Plugin is loaded!");
}

To check if validation plugin is loaded or not using JQuery.

if(jQuery().validate) {
    alert("Validation plugin exists");
    // Now I can use $('#someId').validate()
}

We can also use jQuery isFunction() to determine if a there're a jQuery UI function.for Ex:- To Check if datepicker exists/ loaded or not

jQuery(document).ready(function() {
if (jQuery.isFunction(jQuery.fn.datepicker)) {
    console.log('datepicker Present')
}
});

JavaScript String slice, substring and substr Methods

JavaScript String holds the sequence of charters.

 var str = "This is Sample String";

To Extract portion of a String in javascript we have 3 methods avilable. They are

String.slice(start, end)
String.substring(start, end)
String.substr(start, length)

String.slice()

This Method Extracts the part of a String and returns the extracted part as String.
It takes 2 arguments for start index and end index.

If we give negative arguments, String.slice method will extract the String from end position to start position
if we give single positive argument, then String.slice extracts the String from that position to the End of the String.
if we give single negative argument, then String.slice extracts the String from End of the String to those many characters specified in the argument.
If we Specify the out of index, It will return the empty String.

See below for Example of String.Slice behavior

 var str = "This is Sample String";
console.log("slice(8,14) : "+str.slice(8,14));
console.log("slice(-13,-7) : "+str.slice(-13,-7));
console.log("slice(8) : "+str.slice(8));
console.log("slice(-7) : "+str.slice(-7));
console.log("slice() : "+str.slice());
console.log("slice(50) : "+str.slice(50));

Output for Above :-

 slice(8,14) : Sample
slice(-13,-7) : Sample
slice(8) : Sample String
slice(-7) :  String
slice() : This is Sample String
slice(50) :

String.substring()

JavaScript substring method is similar to slice, except behavior is different for negative argument.
Unlike Slice method which takes relative index from end of the String for negative arguments, substring method treats them from starting position of the String.
Say if i have a String str = "abc", and i am performaing str.substring(-3,-7), then Javascript will try to substring the str @ index between -13 and -7 and we don't have any characters at that indexes, it will return null.
However, If we use str.subgtring(-3), javaScript tries to return the SubString from -7 index to length of the String and we have String available from 0^th Index, It will return the entire String.

See below for Example of String.substring() behavior

var str = "This is Sample String";
console.log("substring(8,14) : "+str.substring(8,14));
console.log("substring(-13,-7) : "+str.substring(-13,-7));
console.log("substring(8) : "+str.substring(8));
console.log("substring(-7) : "+str.substring(-7));
console.log("substring() : "+str.substring());
console.log("substring(50) : "+str.substring(50));

Output for Above :-

substring(8,14) : Sample
substring(-13,-7) :
substring(8) : Sample String
substring(-7) : This is Sample String
substring() : This is Sample String
substring(50) :

String.substr()

JavaScript substr method is similar to slice, except the second argument specifies the length of the string.
Also Behaviour of subStr() and Slice() methods changes based on the negative argument supplied.
If we pass only one argument and is negative to the subStr(), then it works similar like slice() method to from end of the String but it will return String with the length specified foe Ex:- str.subStr(-7) means it will return the String of 7 characters from End of the String str.
If we pass 2 negative arguments to the substr() method, it will return the String from first argument position from end of the String to the lenght specified in second argument.

See below for Example of String.substr() behavior for clear understanding.

var str = "This is Sample String";
console.log("substr(8,14) : "+str.substr(8,14));
console.log("substr(-11,7) : "+str.substr(-11,7));
console.log("substr(8) : "+str.substr(8));
console.log("substr(-7) : "+str.substr(-7));
console.log("substr() : "+str.substr());
console.log("substr(50) : "+str.substr(50));

Output for Above :-

substr(8,14) : Sample String
substr(-11,7) : mple St
substr(8) : Sample String
substr(-7) :  String
substr() : This is Sample String
substr(50) :

From Above examples, we can conclude that If we want to perform operation based on indices, then we can go for slice() or subString() keeping in mind the behavior of the negative arguments. If we want to perform operation based on length then we go for substr().

For those who want to know the performance of these 3 methods here is a good jsperf comparison. http://jsperf.com/slice-vs-substr-vs-substring-methods-long-string/3

JavaScript Math object Examples and Reference

Math is a built-in JavaScript object that has properties and methods to perform mathematical tasks. The Math object cannot be created using the new operator. It has no constructor defined. All the Properties and Methods of Math object are static. That means we no need to create object for it, we can directly access the properties and methods using Math.

JavaScript Math Properties or Constants

JavaScript Math Object has 8 Constants, They are

Math.E

Euler's constant and the base of natural logarithms, approximately 2.718.
Ex:-

var EulerVal = Math.E;
console.log("Euler's Value is : " + EulerVal);

Output is:

  Euler's Value is : 2.718281828459045

Math.LN2

This returns natural logarithm of 2
Ex:-

var Val = Math.LN2;
console.log("Val's Value is : " + Val);

Output is:

  Val's Value is : 0.6931471805599453

Math.LN10

This returns natural logarithm of 10
Ex:-

var Val = Math.LN10;
console.log("Val's Value is : " + Val);

Output is:

  Val's Value is : 2.302585092994046

Math.LOG2E

This returns base 2 logarithm of E
Ex:-

var Val = Math.LOG2E;
console.log("Val's Value is : " + Val);

Output is:

  Val's Value is : 1.4426950408889634

Math.LOG10E

This returns base 10 logarithm of E
Ex:-

var Val = Math.LOG10E;
console.log("Val's Value is : " + Val);

Output is:

  Val's Value is : 0.4342944819032518

Math.PI

This returns the ratio of the circumference of a circle to its diameter
Ex:-

var Val = Math.PI;
console.log("Val's Value is : " + Val);

Output is:

  Val's Value is : 3.141592653589793

Math.SQRT2

Square root of 2
Ex:-

var Val = Math.SQRT2;
console.log("Val's Value is : " + Val);

Output is:

  Val's Value is : 1.4142135623730951

Math.SQRT1_2

Square root of 1/2
Ex:-

var Val = Math.SQRT1_2;
console.log("Val's Value is : " + Val);

Output is:

  Val's Value is : 0.7071067811865476

JavaScript Math Object Methods

Below are the list of JavaScript Math Object methods their use and examples.

Math.abs(x)

This method returns the absolute value of a number(x)
Ex:-

var val = Math.abs(-1);
console.log("Value is : " + val); 

val = Math.abs(0);
console.log("Value is : " + val); 

val = Math.abs(2);
console.log("Value is : " + val);

Output is:

Value is : 1
Value is : 0
Value is : 2

Math.acos(x)

This method returns returns the arccosine of x, in radians
Returns a numeric value between 0 and PI radians for x between -1 and 1. If the value of number is outside this range, it returns NaN. Ex:-

var val = Math.acos(-1);
console.log("Value is : " + val); 

val = Math.acos(0);
console.log("Value is : " + val); 

val = Math.acos(2);
console.log("Value is : " + val);

Output is:

Value is : 3.141592653589793
Value is : 1.5707963267948966
Value is : NaN

Math.asin(x)

This method returns the arcsine of x, in radians
Returns a numeric value between -pi/2 and pi/2 radians for x between -1 and 1. If the value of number is outside this range, it returns NaN. Ex:-

var val = Math.asin(-1);
console.log("Value is : " + val); 

val = Math.asin(0);
console.log("Value is : " + val); 

val = Math.asin(2);
console.log("Value is : " + val);

Output is:

Value is : -1.5707963267948966
Value is : 0
Value is : NaN

Math.atan(x)

This method returns the arctangent in radians of a number. The atan method returns a numeric value between -pi/2 and pi/2 radians
Ex:-

var val = Math.atan(-1);
console.log("Value is : " + val); 

val = Math.atan(0);
console.log("Value is : " + val); 

val = Math.atan(2);
console.log("Value is : " + val);

Output is:

Value is : -0.7853981633974483
Value is : 0
Value is : 1.1071487177940904

Math.atan2(x,y)

This method returns the arctangent of the quotient of its arguments. The atan2 method returns a numeric value between -pi and pi
Ex:-

var val = Math.atan2(30,60);
console.log("Value is : " + val);

Output is:

Value is : 0.4636476090008061

Math.ceil(x)

This method returns the smallest integer greater than or equal to a number
Ex:-

var val = Math.ceil(1.25);
console.log("Value is : " + val); 

var val = Math.ceil(1.55);
console.log("Value is : " + val);

Output is:

Value is : 2
Value is : 2

Math.cos(x)

This method returns the cosine of a number. The cos method returns a numeric value between -1 and 1
Ex:-

var val = Math.cos(30);
console.log("Value is : " + val); 

var val = Math.cos(60);
console.log("Value is : " + val);

Output is:

Value is : 0.15425144988758405
Value is : -0.9524129804151563

Math.exp(x)

This method returns Ex, where x is the argument, and E is Euler's constant, the base of the natural logarithms
Ex:-

var val = Math.exp(1);
console.log("Value is : " + val); 

var val = Math.exp(2);
console.log("Value is : " + val);

Output is:

Value is : 2.718281828459045
Value is : 7.38905609893065

Math.floor(x)

This method returns the largest integer less than or equal to a number
Ex:-

var val = Math.floor(1.25);
console.log("Value is : " + val); 

var val = Math.floor(1.55);
console.log("Value is : " + val);

Output is:

Value is : 1
Value is : 1

Math.log(x)

This method returns the natural logarithm (base E) of a number. If the value of number is negative, the return value is always NaN
Ex:-

var val = Math.log(1.25);
console.log("Value is : " + val); 

var val = Math.log(1.55);
console.log("Value is : " + val);

Output is:

Value is : 0.22314355131420976
Value is : 0.4382549309311553

Math.max(val1, val2, ... valN)

This method returns the largest of zero or more numbers. If no arguments are given, the results is Infinity
Ex:-

var val = Math.max(1.25, 2.5);
console.log("Value is : " + val); 

var val = Math.max(1, 2, 5);
console.log("Value is : " + val); 

var val = Math.max();
console.log("Value is : " + val);

Output is:

Value is : 2.5
Value is : 5
Value is : -Infinity

Math.min(val1, val2, ... valN)

This method returns the smallest of zero or more numbers. If no arguments are given, the results is Infinity
Ex:-

var val = Math.min(1.25, 2.5);
console.log("Value is : " + val); 

var val = Math.min(1, 2, 5);
console.log("Value is : " + val); 

var val = Math.min();
console.log("Value is : " + val);

Output is:

Value is : 1.25
Value is : 1
Value is : Infinity

Math.pow(base, exponent )

This method returns the base to the exponent power
Ex:-

var val = Math.pow(1.25, 2.5);
console.log("Value is : " + val); 

var val = Math.pow(1, 2);
console.log("Value is : " + val); 

var val = Math.pow();
console.log("Value is : " + val);

Output is:

Value is : 1.7469281074217107
Value is : 1
Value is : NaN

Math.random()

This method returns a random number between 0 (inclusive) and 1 (exclusive)
Ex:-

var val = Math.random();
console.log("Value is : " + val);

Output is:

Value is : 0.49192287446931005

Math.round(x)

This method returns the value of a number rounded to the nearest integer
Ex:-

var val = Math.round(1.25);
console.log("Value is : " + val); 

var val = Math.round(1.55);
console.log("Value is : " + val);

Output is:

Value is : 1
Value is : 2

Math.sin(x)

This method returns the sine of a number. The sin method returns a numeric value between -1 and 1
Ex:-

var val = Math.sin(1);
console.log("Value is : " + val); 

var val = Math.sin(60);
console.log("Value is : " + val);

Output is:

Value is : 0.8414709848078965
Value is : -0.3048106211022167

Math.sqrt(x)

This method returns the square root of a number. If the value of number is negative, sqrt returns NaN
Ex:-

var val = Math.sqrt(24);
console.log("Value is : " + val); 

var val = Math.sqrt(-24);
console.log("Value is : " + val);

Output is:

Value is : 4.898979485566356
Value is : NaN

Math.tan(x)

This method returns the tangent of a number. The tan method returns a numeric value that represents the tangent of the angle
Ex:-

var val = Math.tan(30);
console.log("Value is : " + val); 

var val = Math.tan(90);
console.log("Value is : " + val);

Output is:

Value is : -6.405331196646276
Value is : -1.995200412208242