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)