mistake:
An error was reported when Bootstrap was introduced into the recent project: Uncaught ReferenceError: Popper is not defined
reason:
There is a reference to Popper.js in bootstrap.bundle.js in Bootstrap 4, so you need to refer to the Popper.js file before introducing Bootstrap
It should be noted that it must be the UMD version of the Popper.js file, otherwise an error will be reported: Uncaught SyntaxError: Unexpected token export
popper.js download address: https://cdnjs.com/libraries/popper.js
Bootstrap v4.1.0 select 1.14.0 popper.js
