MDBitz Development Resources

When developing within a large web application it may be needed to check if a JavaScript function/object exists prior to being calling. The benefits to checking if the method exists are:

1. No JavaScript errors are Generated.
2. if needed you can dynamically load and initialize your method or object

In most cases you won’t need to test for a function but in the rare cases that you find yourself not sure of what has been loaded into the DOM (Document Object Model) you can check by simply calling window.methodName. By wrapping it in a if statement it will let you know if the method exists.

if( window.methodname ) {
  // method exists
} else {
  // method does not exist
  function methodName() {
 
  }
}

Javascript Cookies are variables that are stored on the visiting users computer. These cookies are unique to a domain so that if the site www.example.com sets a cookie with information in it that cookie can not be read and utilized by www.xyz.com. In most cases cookies are used to save user settings for your website, when they last logged in, etc. However they should not be used to store sensitive information as the cookie information is transferred with each page request to the server.

With that out of the way lets get right into things, and understand the 3 things that define your cookie value.

• Name: The name to identify your cookie
• Value: The value to be stored in the cookie
• Expiration Date: When does this cookie value expire

The cookie is set and read by using JavaScript and in particular the document.cookie element of the DOM. You set a cookie by setting document.cookie to your desired name-value pair and expiration date with the format of “name=value; expires=date;”. For example if we wanted to define a cookie with the property user and set it to abc123 with an expiration day of March 3s^st

document.cookie ='user=abc123; expires=Wed, 31 Mar 2010 20:47:11 UTC;

To access the saved cookie we would read out the value of document.cookie. This property returns a string representation of all the name-value properties that have been set to the cookie. Therefore you will need to parse the returned String to get your desired value. The easiest way to do this is to get the indexOf method to get the starting point of the property value and get the value up to the next semi-colon or end of the string if one doesn’t exist.

To help illustrate lets assume I have set a cookie with user = abc123 and settings = Show All. If a cookie was set with these properties then document.cookie would return the following.

user=abc123; settings=Show All

Therefore to get the user property you would need to get the index of user= and return the string from that index plus its length until the next ; or end of the cookie value if user was the last property.

start_idx=document.cookie.indexOf("user=");
cookie_value = "";
if (start_idx != -1)
{
    start_idx=start_idx+ 6;
    end_idx=document.cookie.indexOf(";" , start_idx);
    if (end_idx == -1) {
        end_idx = document.cookie.length;
    }
    cookie_value = document.cookie.substring(start_idx, end_idx);
}
alert(cookie_value);

To make things easy on yourself, there are a lot of existing functions and libraries that have defined methods for reading and writing cookies. W3Schools has an excellent set of functions at www.w3schools.com/JS/js_cookies.asp. In addition many JavaScript libraries either have built in cookie support or plugins for handling them.

In Dynamic websites we often find that we need to modify page content on the fly based on user actions. A common example of this is allowing the user to add additional input boxes for configurations or attachments. I personally like to contain such input as a new row in a table. To accomplish this I utilize the jQuery library for JavaScript.

For illustration lets assume we have the following table in our page:

<table id="exampleTable1">
<tr>
    <th>City</th><th>State</th>
</tr>
<tr>
    <td>Rochester</td><td>New York</td>
</tr>
<tr>
    <td>Boston</td><td>Massachusetts</td>
</tr>
</table>

In this example the approach to adding more columns is very straightforward as we can utilize jQuery’s child and last selectors with the after modifier to add a new table row.

var newRow = '<tr><td>Albany</td><td>New York</td></tr>';
$('#exampleTable1 tr:last').after(newRow);

These 2 lines of JavaScript create the new row as a string then utilizes jQuery to get the last row of the table and append the new row after it, adding this row to the end of our table.

If a table contains a tfoot element with rows then the above will insert a row in the footer, A modification that can be done is to obtain the tbody element of the table and append the new row.

$('#exampleTable1 tbody').append(newRow);

That is all there is to it simply define your new row as a string then utilize jQuery to identify where you would like that row added to the DOM.