How to close electron app via javascript?

I am running an express app via electron.

Below is the main.js

   const electron = require("electron"),
          app = electron.app,
          BrowserWindow = electron.BrowserWindow;

    let mainWindow;

    function createWindow () {
      mainWindow = new BrowserWindow({
          width: 1200,
        height: 800,
        frame: false,
        kiosk: true
      });
      mainWindow.loadURL(`file://${__dirname}/index.html`)
     mainWindow.webContents.openDevTools();

      mainWindow.on("closed", function () {
        mainWindow = null;
      })
    }

    app.on("ready", createWindow);
    app.on("browser-window-created",function(e,window) {
      window.setMenu(null);
    });

    app.on("window-all-closed", function () {
      if (process.platform !== "darwin") {
        app.quit();
      }
    });



    app.on("activate", function () {
      if (mainWindow === null) {
        createWindow();
      }
    });

and below is a button in the view that upon click i would like it to close the app

<button id="close-btn"><i class="fa fa-cube" aria-hidden="true"></i>&nbsp; Close application</button>

Essentially I want to activate this part of the code once the button has been clicked

  app.on("window-all-closed", function () {
          if (process.platform !== "darwin") {
            app.quit();
          }
        });

via John