I have created a custom detail page button(onclick calling Javascript) that is showing in salesforce classic experience but not in lightning experience. I have added the button to the page layout. I don't understand why this is happening. Can some one explain it to me ?
There are screenshots of the detail pages :
Lightning:
Classic :
Actually when the record is saved that means it shows the button in detail page if it is missed then check once again and do the process clearly in page layout and save it not do quick save do full save it definitely shows ...........!
As per my comment, make sure you VF page is marked as Available for Salesforce1 and Lightning. Then the second thing you need to do is override the Salesforce1 and Lightning Experience Actions section in your page layout and add your button there instead of the normal place. You will will find your button available in the Salesforce1 Actions on the top of your screen. The order of your button list will define how it will be displayed in Lightning.
Best Answer
I think you can not call JS from a button in Lightning.
When transitioning from classic to lightning one of the things you should take care is exactly that.
SF considers calling JS from a button to be security risk and hard enforce good coding practices there.
You should design the same functionality using hopefully Process or Flow. If they are not sufficient try with a custom apex action in Flow to do what you need there.
Or maybe custom Lightning component to make easier transition if you use Js a lot for your functionality.