Stuck with OOP Class

Question

Hello I'm trying to run this code in the OOP Class framework however there's this error of "Cannot read properties of undefined (reading 'join').

Why is that?

class Window {
  constructor(tabs) {
    this.tabs = tabs;
  }
  join(otherWindow) {
    const {tabs} = this;
    tabs = tabs.concat(otherWindow.tabs)
  }
  tabOpen() {
    const {tabs} = this;
    tabs.push('new tab')
  }
  tabClose(index) {
    const {tabs} = this;
    const tabsBeforeIndex = tabs.slice(0, index)
    const tabsAfterIndex = tabs.slice(index+1)
    tabs = tabsBeforeIndex.concat(tabsAfterIndex)
  }
}

const workWindow = new Window(['GMail', 'Inbox', 'Work mail', 'Docs', 'freeCodeCamp'])
const socialWindow = new Window(['FB', 'Gitter', 'Reddit', 'Twitter', 'Medium']);
const videoWindow = new Window(['Netflix', 'YouTube', 'Vimeo', 'Vine']);

const finalTabs = socialWindow.tabOpen().join(videoWindow.tabClose(2)).join(workWindow.tabClose(1).tabOpen());

console.log(finalTabs)