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16.4. Cookie Example

Example 16-1 brings together all the aspects of cookies we have discussed so far. First, the example defines a Cookie class. When you create a Cookie object, you specify a Document object, a name for the cookie, and, optionally, an expiration time, a path, a domain, and a boolean value that indicates whether the cookie should be secure. After creating a Cookie object, you can set arbitrary string properties on this object; the values of these properties are the values stored in the cookie.

The Cookie class defines three methods. The store( ) method loops through all of the user-defined properties of the Cookie object and concatenates their names and values into a single string that serves as the value of the cookie. The load( ) method of a Cookie object reads the cookie property of the Document object to obtain the values of all the cookies for the document. It searches this string to find the value of the named cookie and then parses this value into individual names and values, which it stores as properties of the Cookie object. Finally, the remove( ) method of the Cookie object deletes the specified cookie from the document.

After defining the Cookie class, Example 16-1 demonstrates a useful and elegant way to use cookies. The code is somewhat complicated but is worth studying carefully. You may want to start with the test program at the end of the example; it shows a typical usage of the Cookie class.

Example 16-1. A utility class for working with cookies

<script language="JavaScript1.1">
// The constructor function: creates a Cookie object for the specified
// document, with a specified name and optional attributes.
// Arguments:
//   document: The Document object for which the cookie is stored. Required.
//   name:     A string that specifies a name for the cookie. Required.
//   hours:    An optional number that specifies the number of hours from now
//             after which the cookie should expire.
//   path:     An optional string that specifies the cookie path attribute.
//   domain:   An optional string that specifies the cookie domain attribute.
//   secure:   An optional boolean value that, if true, requests a secure cookie.
//
function Cookie(document, name, hours, path, domain, secure)
{
    // All the predefined properties of this object begin with '$'
    // to distinguish them from other properties, which are the values to
    // be stored in the cookie
    this.$document = document;
    this.$name = name;
    if (hours)
        this.$expiration = new Date((new Date()).getTime( ) + hours*3600000);
    else this.$expiration = null;
    if (path) this.$path = path; else this.$path = null;
    if (domain) this.$domain = domain; else this.$domain = null;
    if (secure) this.$secure = true; else this.$secure = false;
}

// This function is the store( ) method of the Cookie object
Cookie.prototype.store = function ( ) {
    // First, loop through the properties of the Cookie object and
    // put together the value of the cookie. Since cookies use the
    // equals sign and semicolons as separators, we'll use colons
    // and ampersands for the individual state variables we store 
    // within a single cookie value. Note that we escape the value
    // of each state variable, in case it contains punctuation or other
    // illegal characters.
    var cookieval = "";
    for(var prop in this) {
        // Ignore properties with names that begin with '$' and also methods
        if ((prop.charAt(0) == '$') || ((typeof this[prop]) == 'function')) 
            continue;
        if (cookieval != "") cookieval += '&';
        cookieval += prop + ':' + escape(this[prop]);
    }

    // Now that we have the value of the cookie, put together the 
    // complete cookie string, which includes the name and the various
    // attributes specified when the Cookie object was created
    var cookie = this.$name + '=' + cookieval;
    if (this.$expiration)
        cookie += '; expires=' + this.$expiration.toGMTString( );
    if (this.$path) cookie += '; path=' + this.$path;
    if (this.$domain) cookie += '; domain=' + this.$domain;
    if (this.$secure) cookie += '; secure';

    // Now store the cookie by setting the magic Document.cookie property
    this.$document.cookie = cookie;
}

// This function is the load( ) method of the Cookie object
Cookie.prototype.load = function( ) { 
    // First, get a list of all cookies that pertain to this document
    // We do this by reading the magic Document.cookie property
    var allcookies = this.$document.cookie;
    if (allcookies == "") return false;

    // Now extract just the named cookie from that list
    var start = allcookies.indexOf(this.$name + '=');
    if (start == -1) return false;   // Cookie not defined for this page
    start += this.$name.length + 1;  // Skip name and equals sign
    var end = allcookies.indexOf(';', start);
    if (end == -1) end = allcookies.length;
    var cookieval = allcookies.substring(start, end);

    // Now that we've extracted the value of the named cookie, we
    // must break that value down into individual state variable 
    // names and values. The name/value pairs are separated from each
    // other by ampersands, and the individual names and values are
    // separated from each other by colons. We use the split( ) method
    // to parse everything.
    var a = cookieval.split('&');    // Break it into an array of name/value pairs
    for(var i=0; i < a.length; i++)  // Break each pair into an array
        a[i] = a[i].split(':');

    // Now that we've parsed the cookie value, set all the names and values
    // of the state variables in this Cookie object. Note that we unescape( )
    // the property value, because we called escape( ) when we stored it.
    for(var i = 0; i < a.length; i++) {
        this[a[i][0]] = unescape(a[i][1]);
    }

    // We're done, so return the success code
    return true;
}

// This function is the remove( ) method of the Cookie object
Cookie.prototype.remove = function( ) {
    var cookie;
    cookie = this.$name + '=';
    if (this.$path) cookie += '; path=' + this.$path;
    if (this.$domain) cookie += '; domain=' + this.$domain;
    cookie += '; expires=Fri, 02-Jan-1970 00:00:00 GMT';

    this.$document.cookie = cookie;
}


//===================================================================
//  The previous code is the definition of the Cookie class.
//  The following code is a sample use of that class.
//===================================================================

// Create the cookie we'll use to save state for this web page.
// Since we're using the default path, this cookie will be accessible
// to all web pages in the same directory as this file or "below" it.
// Therefore, it should have a name that is unique among those pages.
// Note that we set the expiration to 10 days in the future.
var visitordata = new Cookie(document, "name_color_count_state", 240);

// First, try to read data stored in the cookie. If the cookie is not
// defined, or if it doesn't contain the data we need, then query the
// user for that data.
if (!visitordata.load( ) || !visitordata.name || !visitordata.color) {
    visitordata.name = prompt("What is your name:", "");
    visitordata.color = prompt("What is your favorite color:", "");
}

// Keep track of how many times this user has visited the page:
if (visitordata.visits == null) visitordata.visits = 0;
visitordata.visits++;

// Store the cookie values, even if they were already stored, so that the 
// expiration date will be reset to 10 days from this most recent visit.
// Also, store them again to save the updated visits state variable.
visitordata.store( );

// Now we can use the state variables we read:
document.write('<font size="7" color="' + visitordata.color + '">' +
               'Welcome, ' + visitordata.name + '!' +
               '</font>' +
               '<p>You have visited ' + visitordata.visits + ' times.');
</script>

<form>
<input type="button" value="Forget My Name" onclick="visitordata.remove( );">
</form>



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