Dropdowns are toggleable, contextual overlays for displaying lists of links and more. They’re made interactive with the included Bootstrap dropdown JavaScript plugin.
Learn more about dropdown in Bootstrap 4 click here.

Similarly, create split button dropdowns with virtually the same markup as single button dropdowns, but with the addition of .dropdown-toggle-split for proper spacing around the dropdown caret. We use this extra class to reduce the horizontal padding on either side of the caret by 25% and remove the margin-left that’s added for regular button dropdowns. Those extra changes keep the caret centered in the split button and provide a more appropriately sized hit area next to the main button.

Trigger dropdown menus above elements by adding .dropup to the parent element.