How to use the @lumino/widgets.BoxLayout function in @lumino/widgets

this.addClass(APPLICATION_SHELL_CLASS);
    this.id = 'main';

    let headerPanel = (this._headerPanel = new Panel());
    let topHandler = (this._topHandler = new Private.PanelHandler());
    let bottomPanel = (this._bottomPanel = new BoxPanel());
    let hboxPanel = new BoxPanel();
    let dockPanel = (this._dockPanel = new DockPanelSvg({
      kind: 'dockPanelBar'
    }));
    MessageLoop.installMessageHook(dockPanel, this._dockChildHook);

    let hsplitPanel = new SplitPanel();
    let leftHandler = (this._leftHandler = new Private.SideBarHandler());
    let rightHandler = (this._rightHandler = new Private.SideBarHandler());
    let rootLayout = new BoxLayout();

    headerPanel.id = 'jp-header-panel';
    topHandler.panel.id = 'jp-top-panel';
    bottomPanel.id = 'jp-bottom-panel';
    hboxPanel.id = 'jp-main-content-panel';
    dockPanel.id = 'jp-main-dock-panel';
    hsplitPanel.id = 'jp-main-split-panel';

    leftHandler.sideBar.addClass(SIDEBAR_CLASS);
    leftHandler.sideBar.addClass('jp-mod-left');
    leftHandler.stackedPanel.id = 'jp-left-stack';

    rightHandler.sideBar.addClass(SIDEBAR_CLASS);
    rightHandler.sideBar.addClass('jp-mod-right');
    rightHandler.stackedPanel.id = 'jp-right-stack';

constructor(options: MainAreaWidget.IOptions) {
    super(options);
    this.addClass('jp-MainAreaWidget');
    this.id = DOMUtils.createDomID();

    const content = (this._content = options.content);
    const toolbar = (this._toolbar = options.toolbar || new Toolbar());
    const spinner = this._spinner;

    const layout = (this.layout = new BoxLayout({ spacing: 0 }));
    layout.direction = 'top-to-bottom';
    BoxLayout.setStretch(toolbar, 0);
    BoxLayout.setStretch(content, 1);
    layout.addWidget(toolbar);
    layout.addWidget(content);

    if (!content.id) {
      content.id = DOMUtils.createDomID();
    }
    content.node.tabIndex = -1;

    this._updateTitle();
    content.title.changed.connect(this._updateTitle, this);
    this.title.closable = true;
    this.title.changed.connect(this._updateContentTitle, this);