Deletes all group members from a facebook group except yourself. Leaving the group after running this in the javascript console will delete the group. Tested on Chrome May 29, 2017. If your network is super slow, you might want to increase the timeout delay.

facebook-delete-all-group-members.js

// removes your box from the page so you don't get removed from the group
$$('[data-name=GroupProfileGridItem]')[0].remove();
 
// member action dropdowns
const actions = $$('.adminActions');
 
// open all dropdowns so they're added to the DOM
for (var i = 0; i < actions.length; i++) {
  $$('.adminActions button')[i] && $$('.adminActions button')[i].click();
}
 
// leave group buttons
const buttons = $$('[data-testid=leave_group]');

var i = 0;

function removeAll() {
  setTimeout(function () {
    // click each button
    buttons[i].click();
    // click confirm button with a half second delay
    setTimeout(function() {
      document.getElementsByClassName('layerConfirm')[0].click() 
    }, 500);
    i++;
    if (i < buttons.length) {
      // each time this is called, there will be a 3 second delay
      removeAll();
    }
  }, 3000);
}

removeAll();