How do you detect a mobile device with javascript?

I know that most people try to avoid Javascript mobile detection because not all mobile browsers support 开发者_StackOverflowit, but I am using it in a Javascript function so if the browser doesn't support Javascript, it doesn't matter if it's mobile since it won't be able to use the function. I don't want it to redirect; just to return whether the browser is mobile or not. Any ideas?

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var isTouchDevice =  ('ontouchstart' in window) || window.DocumentTouch && document instanceof DocumentTouch;

in modernizr ,it is like this.

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See: Standard way to detect mobile browsers in a web application based on the http request

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You can use handsetdetection.com to detect mobile browsers in javascript. We do a detection based on the http headers and send javascript to the client. (disclaimer, i work there).