Bootstrap 4: Uncaught ReferenceError: Popper is not defined

Since our Bootstrap beta 2 release, we added two new dist files : bootstrap.bundle.js and bootstrap.bundle.min.js which contain Popper.js inside

Plus use this link to find the latest release of Popper.js : https://cdnjs.com/libraries/popper.js because the above linked release (1.8.2) is very old, latest is 1.12.9

BTW you should choose the UMD release of Popper.js because it's the one used by Bootstrap

Use bootstrap.bundle.js that already has popper.js NPM path 'bootstrap/dist/js/bootstrap.bundle.js'.