javascript

Chapter 9: Client Detection
}();
return {
engine: engine
};
In this code, a global variable named client is declared to hold the information. Within the anonymous
function is a local variable named engine that contains an object literal with some default settings.
Each rendering engine is represented by a property that is set to 0. If a particular engine is detected,
the version of that engine will be placed into the corresponding property as a floating - point value. The
full version of the rendering engine (a string) is placed into the ver property. This setup allows code
such as the following:
if (client.engine.ie) { //if it’s IE, client.ie is greater than 0
//IE-specific code
} else if (client.engine.gecko > 1.5){
if (client.engine.ver == “1.8.1”){
//do something specific to this version
}
}
Whenever a rendering engine is detected, its property on client.engine gets set to a number greater
than 0, which converts to a Boolean true . This allows a property to be used with an if statement to
determine the rendering engine being used even if the specific version isn ’ t necessary. Since each
property contains a floating - point value, it ’ s possible that some version information may be lost. For
instance, the string ” 1.8.1 ” becomes the number 1.8 when passed into parseFloat() . The ver
property assures that the full version is available if necessary.
To identify the correct rendering engine, it ’ s important to test in the correct order. Testing out of order
may result in incorrect results due to the user - agent inconsistencies. For this reason, the first step is to
identify Opera, since its user - agent string may completely mimic other browsers. Opera ’ s user - agent
string cannot be trusted since it won ’ t, in all cases, identify itself as Opera.
To identify Opera, it ’ s necessary to look for the window.opera object. This object is present in all
versions of Opera 5 and later, and is used to identify information about the browser and to interact
directly with the browser. In versions later than 7.6, a method called version() returns the
browser version number as a string, which is the best way to determine the Opera version number.
Earlier versions may be detected using the user - agent string, since identity masking wasn ’ t supported.
However, since Opera ’ s most recent version at the end of 2007 was 9.5, it ’ s unlikely that anyone is using
a version older than 7.6. The first step in the rendering engine ’ s detection code is as follows:
if (window.opera){
engine.ver = window.opera.version();
engine.opera = parseFloat(client.ver);
}
The string representation of the version is stored in engine.ver , and the floating - point representation is
stored in engine.opera . If the browser is Opera, the test for window.opera will return true .
Otherwise, it ’ s time to detect another browser.
The next logical rendering engine to detect is WebKit. Since WebKit ’ s user - agent string contains ” Gecko ”
and ” KHTML ” , incorrect results could be returned if you were to check for those rendering engines first.
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