JavaScript

All of the maths functions in JavaScript are kept in a handy object called Math, which contains a number of different functions.

To get the absolute value of a number use the abs() function.

Math.abs(3.14159265) // returns 3.14159265

Rounding a number is done by either the round() function to round to the nearest integer, the ceil() function to round up to the nearest integer and the floor() function to round down to the nearest integer.

Math.ceil(3.14159265)    // returns 4
Math.floor(3.14159265)   // returns 3
Math.round(3.14159265) // returns 3

To find the exponent of a number use the exp() function.

Math.exp(3.14159265) // returns 23.140692549708973

The log() method returns the natural logarithm (base E) of a number.

To set up multiple Google Analytics tags on the same page you need to use the _uff = false; command in between the unchinTracker() calls to reset the tracker for the next account. The urchinTracker() function will send information on the page visit off to Google Analytics.

<script src="http://www.google-analytics.com/urchin.js" type="text/javascript"></script>
<script type="text/javascript">
  _uacct = "UA-XXXXX"; // First account details
  urchinTracker();
  _uff = 0; // Reset tracker for second account
  _uacct = "UA-YYYYY"; // Second account details
  urchinTracker();
</script>

You can do this for as many accounts as you like, but be aware that there will come a point when there will be a noticeable delay on the site when the calls to Google Analytics are done so don't do too many.

Creating user interfaces in JavaScript can sometimes lead to a problem, especially when the interface used AJAX to load data from the server. As many actions will be event driven by the user you can find that when a user triggers lots of events all at once the browser will send out lots of AJAX requests all at once. This can easily cause bandwidth issues but can also lead to the user getting confused while they patiently await the browser to settle down and let them get on with things.

There are numerous way to get around this issue, the first being to design programs so that AJAX requests are only issued when the user has clicked on something or when they start typing

The for loop in JavaScript can be used to iterate through all items in an array or properties of an object. This makes looping through any object or array very easy. As in the following example that printing out all items in an array.

var count = '';
var numbers = new Array(3);
numbers[0] = 42;
numbers[1] = 13;
numbers[2] = 73;
for(i in numbers){
  count += numbers[i]+' ';
}
alert(count);

The three main objects to do with a JavaScript are navigator, window and document. Although window is part of navigator and document is part of window. So to print off all information to do with a browser you can do the following. Note that some items in window and document can cause JavaScript to crash in some browsers so some error detection is included here.

Here are a few of the built in JavaScript functions available.

To get the length of a string use the length variable. This returns the number of characters in a string.

var str = 'abcdef';
str.length; // returns 6

The charAt function will return the character at the point specified in the parameter. So to get the first character of a string use:

var str = 'abcdef';
str.charAt(0); // returns a

To get the last character of a string use a combination of charAt and length.

var str = 'abcdef';
str.charAt(str.length - 1); // returns 'f'

To get the position of a character or group of characters in a string use the indexOf function. The function returns -1 if the string is not found.

To include a JavaScript file into a HTML page you can use the script tag with the src parameter pointing towards the source code file. However, there is a subtle difference between the script tag in HTML and XHTML. This is because the language attribute is not supported in XHTML, so if you just copy the code from HTML to XHTML the page won't validate. The solution here is to just leave it out.

For HTML

<script language="JavaScript" type="text/javascript" src="scripts/javascript.js"></script>

For XHTML

<script type="text/javascript" src="scripts/javascript.js"></script>

The type attribute is the mime type of the script, which is always text/javascript. Although there is come discussion about what the mime type is supposed to be, best results are usually obtained by just sticking with text/javascript.

Small JavaScript function to check if a variable is a number.