http://www.ruanyifeng.com/blog/2011/05/how_to_judge_the_existence_of_a_global_object_in_javascript.html
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http://www.ruanyifeng.com/blog/2011/05/how_to_judge_the_existence_of_a_global_object_in_javascript.html
转载于:https://www.cnblogs.com/njqa/p/7345865.html