如何检测是否安装了 Flash，如果没有，如何显示一个隐藏的 div 来通知用户？

Use swfobject. it replaces a div with the flash if it is installed. see: http://code.google.com/p/swfobject/

You can use navigator.mimeTypes.

if (navigator.mimeTypes ["application/x-shockwave-flash"] == undefined)
$("#someDiv").show ();

If swfobject won't suffice, or you need to create something a little more bespoke, try this:

var hasFlash = false;
try {
hasFlash = Boolean(new ActiveXObject('ShockwaveFlash.ShockwaveFlash'));
} catch(exception) {
hasFlash = ('undefined' != typeof navigator.mimeTypes['application/x-shockwave-flash']);
}

It works with 7 and 8.

I used Adobe's detection kit, originally suggested by justpassinby. Their system is nice because it detects the version number and compares it for you against your 'required version'

One bad thing is it does an alert showing the detected version of flash, which isn't very user friendly. All of a sudden a box pops up with some seemingly random numbers.

Some modifications you might want to consider:

• remove the alert
• change it so it returns an object (or array) --- first element is boolean true/false for "was the required version found on user's machine" --- second element is the actual version number found on user's machine

jqplugin: http://code.google.com/p/jqplugin/

$.browser.flash == true

@Drewid's answer didn't work in my Firefox 25 if the flash plugin is just disabled but installed.

@invertedSpear's comment in that answer worked in firefox but not in any IE version.

So combined both their code and got this. Tested in Google Chrome 31, Firefox 25, IE 8-10. Thanks Drewid and invertedSpear :)

var hasFlash = false;
try {
var fo = new ActiveXObject('ShockwaveFlash.ShockwaveFlash');
if (fo) {
hasFlash = true;
}
} catch (e) {
if (navigator.mimeTypes
&& navigator.mimeTypes['application/x-shockwave-flash'] != undefined
&& navigator.mimeTypes['application/x-shockwave-flash'].enabledPlugin) {
hasFlash = true;
}
}

You should also be able to use..

swfobject.getFlashPlayerVersion().major === 0

with the swfobject-Plugin.

Very very minified version of http://www.featureblend.com/javascript-flash-detection-library.html (only boolean flash detection)

var isFlashInstalled = (function(){
var b=new function(){var n=this;n.c=!1;var a="ShockwaveFlash.ShockwaveFlash",r=[{name:a+".7",version:function(n){return e(n)}},{name:a+".6",version:function(n){var a="6,0,21";try{n.AllowScriptAccess="always",a=e(n)}catch(r){}return a}},{name:a,version:function(n){return e(n)}}],e=function(n){var a=-1;try{a=n.GetVariable("$version")}catch(r){}return a},i=function(n){var a=-1;try{a=new ActiveXObject(n)}catch(r){a={activeXError:!0}}return a};n.b=function(){if(navigator.plugins&&navigator.plugins.length>0){var a="application/x-shockwave-flash",e=navigator.mimeTypes;e&&e[a]&&e[a].enabledPlugin&&e[a].enabledPlugin.description&&(n.c=!0)}else if(-1==navigator.appVersion.indexOf("Mac")&&window.execScript)for(var t=-1,c=0;c<r.length&&-1==t;c++){var o=i(r[c].name);o.activeXError||(n.c=!0)}}()};
return b.c;
})();


if(isFlashInstalled){
// Do something with flash
}else{
// Don't use flash
}