JavaScript-How do I play next when I click the Next button in the JS code below

I did a random play, and then to do a bad traversal of the data returned by the return of the song, the effect is to do is to click the left and right button to play the previous and next song, but the result is to click on the left button directly play the first song, and click on the button to play directly on the last song, who help to give a solution

    
  
   
      
  
   
      
  
   
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Reply content:

I did a random play, and then to do a bad traversal of the data returned by the return of the song, the effect is to do is to click the left and right button to play the previous and next song, but the result is to click on the left button directly play the first song, and click on the button to play directly on the last song, who help to give a solution

    
  
   
      
  
   
      
  
   
      <title></title>    
  
   
                                                                                                             
  
  
 
   
 
    
 
    
 
   
 
  
                     
  
  
 
   
 
    
 
    
 
    
 
   
 
  
                                                                                 
  
  
 
   
 
    
Song list
 
   
 
  
                                          

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