Checking Salesforce Classic or Salesforce LEX using javascript

Simple as this :

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var isClassic ='false'; console.log ('@@@ UIThemeDisplayed '+ ' {!$User.UIThemeDisplayed}'); if('{!$User.UIThemeDisplayed} ' == 'Theme3') { isClassic='true'; }

To use visualforce page within Salesforce LEX is to use this url . Some scenario that you might want to apply this is when you want to redirect to Visualforce page in Lightning.

1	/one/one.app#/alohaRedirect/apex/apexpagename?parameter