ieUserAgent.js

var ieUserAgent = {
    init: function () {
        // Get the user agent string
        var ua = navigator.userAgent;
        this.compatibilityMode = false;
        
        // Detect whether or not the browser is IE
        var ieRegex = new RegExp("MSIE ([0-9]{1,}[\.0-9]{0,})");
        if (ieRegex.exec(ua) == null)
            this.exception = "The user agent detected does not contai Internet Explorer.";

        // Get the current "emulated" version of IE
        this.renderVersion = parseFloat(RegExp.$1);
        this.version = this.renderVersion;

        // Check the browser version with the rest of the agent string to detect compatibility mode
        if (ua.indexOf("Trident/6.0") > -1) {
            if (ua.indexOf("MSIE 7.0") > -1) {
                this.compatibilityMode = true;
                this.version = 10;
            }
        }
        else if (ua.indexOf("Trident/5.0") > -1) {      
            if (ua.indexOf("MSIE 7.0") > -1) {
                this.compatibilityMode = true;
                this.version = 9;
            }
        }
        else if (ua.indexOf("Trident/4.0") > -1) {
            if (ua.indexOf("MSIE 7.0") > -1) {
                this.compatibilityMode = true;
                this.version = 8;
            }
        }
        else if (ua.indexOf("MSIE 7.0") > -1)
            this.version = 7;
        else
            this.version = 6;
    }
};

// Initialize the ieUserAgent object
ieUserAgent.init();

// Do stuff
if (ieUserAgent.compatibilityMode == true){
    if(ieUserAgent.version === 8){
        //trigger alert with ie8 instructions
    }
    if(ieUserAgent.version === 9){
        //trigger alert with ie9 instructions
    }
}