problem:
Calling the drop-down menu with js in Bootstrap4, the error will be reported as follows:
The reason is that there is no Popper.js, we can import it.
Solution:
1. Download node
2. Open the command line to the specified directory, use npm to import, npm install --save Popper.js.
3. Introduce Popper.js into the HTML document, the structure directory of the Popper just downloaded should be like this:
You should import: node_modules \ popper.js \ dist \ umd \ popper.js
4. You must put popper.js on top of bootstrap.js, like this:
