How to use the cesium.defined function in cesium

this.eventHelper.add(this.terria.afterWidgetChanged, function () {
    if (defined(this._removeSubscription)) {
      this._removeSubscription()
      this._removeSubscription = undefined
    }
  }, this)
  //        this.terria.beforeWidgetChanged.addEventListener(function () {

// Register custom Knockout.js bindings.  If you're not using the TerriaJS user interface, you can remove this.
  registerKnockoutBindings()

  if (!defined(this.terria.options.enableDistanceLegend) || this.terria.options.enableDistanceLegend) {
    this.distanceLegendDiv = document.createElement('div')
    container.appendChild(this.distanceLegendDiv)
    this.distanceLegendDiv.setAttribute('id', 'distanceLegendDiv')
    this.distanceLegendViewModel = DistanceLegendViewModel.create({
      container: this.distanceLegendDiv,
      terria: this.terria,
      mapElement: container,
      enableDistanceLegend: true
    })
  }

  if ((!defined(this.terria.options.enableZoomControls) || this.terria.options.enableZoomControls) && (!defined(this.terria.options.enableCompass) || this.terria.options.enableCompass)) {
    this.navigationDiv = document.createElement('div')
    this.navigationDiv.setAttribute('id', 'navigationDiv')
    container.appendChild(this.navigationDiv)
    // Create the navigation controls.
    this.navigationViewModel = NavigationViewModel.create({
      container: this.navigationDiv,
      terria: this.terria,
      enableZoomControls: true,
      enableCompass: true
    })
  } else if ((defined(this.terria.options.enableZoomControls) && !this.terria.options.enableZoomControls) && (!defined(this.terria.options.enableCompass) || this.terria.options.enableCompass)) {
    this.navigationDiv = document.createElement('div')
    this.navigationDiv.setAttribute('id', 'navigationDiv')
    container.appendChild(this.navigationDiv)
    // Create the navigation controls.
    this.navigationViewModel = NavigationViewModel.create({

var NavigationViewModel = function (options) {
  this.terria = options.terria
  this.eventHelper = new EventHelper()
  this.enableZoomControls = (defined(options.enableZoomControls)) ? options.enableZoomControls : true
  this.enableCompass = (defined(options.enableCompass)) ? options.enableCompass : true
  this.navigationLocked = false

  // if (this.showZoomControls)
  //   {
  this.controls = options.controls
  if (!defined(this.controls)) {
    this.controls = [
      new ZoomNavigationControl(this.terria, true),
      new ResetViewNavigationControl(this.terria),
      new ZoomNavigationControl(this.terria, false)
    ]
  }
  // }

  this.svgCompassOuterRing = svgCompassOuterRing
  this.svgCompassGyro = svgCompassGyro

function initialize(viewerCesiumWidget, options) {
  if (!defined(viewerCesiumWidget)) {
    throw new DeveloperError('CesiumWidget or Viewer is required.')
  }

  //        options = defaultValue(options, defaultValue.EMPTY_OBJECT);

  var cesiumWidget = defined(viewerCesiumWidget.cesiumWidget) ? viewerCesiumWidget.cesiumWidget : viewerCesiumWidget

  var container = document.createElement('div')
  container.className = 'cesium-widget-cesiumNavigationContainer'
  cesiumWidget.container.appendChild(container)

  this.terria = viewerCesiumWidget
  this.terria.options = (defined(options)) ? options : {}
  this.terria.afterWidgetChanged = new CesiumEvent()
  this.terria.beforeWidgetChanged = new CesiumEvent()
  this.container = container

if (!sscc.enableTilt || !sscc.enableRotate) {
        return
      }
      break
    case SceneMode.SCENE2D:
      if (!sscc.enableTranslate) {
        return
      }
      break
  }

  // Remove existing event handlers, if any.
  document.removeEventListener('mousemove', viewModel.orbitMouseMoveFunction, false)
  document.removeEventListener('mouseup', viewModel.orbitMouseUpFunction, false)

  if (defined(viewModel.orbitTickFunction)) {
    viewModel.terria.clock.onTick.removeEventListener(viewModel.orbitTickFunction)
  }

  viewModel.orbitMouseMoveFunction = undefined
  viewModel.orbitMouseUpFunction = undefined
  viewModel.orbitTickFunction = undefined

  viewModel.isOrbiting = true
  viewModel.orbitLastTimestamp = getTimestamp()

  var camera = scene.camera

  if (defined(viewModel.terria.trackedEntity)) {
    // when tracking an entity simply use that reference frame
    viewModel.orbitFrame = undefined
    viewModel.orbitIsLook = false

viewModel.rotateIsLook = true
    } else {
      viewModel.rotateFrame = Transforms.eastNorthUpToFixedFrame(viewCenter, scene.globe.ellipsoid, newTransformScratch)
      viewModel.rotateIsLook = false
    }
  }

  var oldTransform
  if (defined(viewModel.rotateFrame)) {
    oldTransform = Matrix4.clone(camera.transform, oldTransformScratch)
    camera.lookAtTransform(viewModel.rotateFrame)
  }

  viewModel.rotateInitialCameraAngle = -camera.heading

  if (defined(viewModel.rotateFrame)) {
    camera.lookAtTransform(oldTransform)
  }

  viewModel.rotateMouseMoveFunction = function (e) {
    var compassRectangle = compassElement.getBoundingClientRect()
    var center = new Cartesian2((compassRectangle.right - compassRectangle.left) / 2.0, (compassRectangle.bottom - compassRectangle.top) / 2.0)
    var clickLocation = new Cartesian2(e.clientX - compassRectangle.left, e.clientY - compassRectangle.top)
    var vector = Cartesian2.subtract(clickLocation, center, vectorScratch)
    var angle = Math.atan2(-vector.y, vector.x)

    var angleDifference = angle - viewModel.rotateInitialCursorAngle
    var newCameraAngle = CesiumMath.zeroToTwoPi(viewModel.rotateInitialCameraAngle - angleDifference)

    var camera = viewModel.terria.scene.camera

    var oldTransform

_.prototype.getGeometry = function() {
      if (!(Cesium.defined(this.center) && Cesium.defined(this.radius))) {
        return
      }

      return new Cesium.CircleGeometry({
        center: this.center,
        radius: this.radius,
        height: this.height,
        vertexFormat: Cesium.EllipsoidSurfaceAppearance.VERTEX_FORMAT,
        stRotation: this.textureRotationAngle,
        ellipsoid: this.ellipsoid,
        granularity: this.granularity,
      })
    }

function _(options) {
      if (
        !(
          Cesium.defined(options.center) &&
          Cesium.defined(options.semiMajorAxis) &&
          Cesium.defined(options.semiMinorAxis)
        )
      ) {
        throw new Cesium.DeveloperError(
          'Center and semi major and semi minor axis are required'
        )
      }

      options = copyOptions(options, defaultEllipseOptions)

      this.initialiseOptions(options)
    }

var DistanceLegendViewModel = function (options) {
  if (!defined(options) || !defined(options.terria)) {
    throw new DeveloperError('options.terria is required.')
  }

  this.terria = options.terria
  this._removeSubscription = undefined
  this._lastLegendUpdate = undefined
  this.eventHelper = new EventHelper()

  this.distanceLabel = undefined
  this.barWidth = undefined

  this.enableDistanceLegend = (defined(options.enableDistanceLegend)) ? options.enableDistanceLegend : true

  Knockout.track(this, ['distanceLabel', 'barWidth'])

  this.eventHelper.add(this.terria.afterWidgetChanged, function () {