How to use JS to determine whether a variable exists

This time I will show you how to use JS to determine whether a variable exists, and what are the precautions for using JS to determine whether a variable exists. The following is a practical case, let's take a look.

     
  //http://www.jb51.net/article/67551.htm //判断变量i是否存在 typeof(i)=="undefined"   

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