How to use the @lumino/widgets.SplitPanel.setStretch function in @lumino/widgets

rendermime,
    manager,
    path,
    contentFactory,
    mimeTypeService: editorServices.mimeTypeService
  });
  consolePanel.title.label = TITLE;

  let palette = new CommandPalette({ commands });

  let panel = new SplitPanel();
  panel.id = 'main';
  panel.orientation = 'horizontal';
  panel.spacing = 0;
  SplitPanel.setStretch(palette, 0);
  SplitPanel.setStretch(consolePanel, 1);
  panel.addWidget(palette);
  panel.addWidget(consolePanel);

  // Attach the panel to the DOM.
  Widget.attach(panel, document.body);

  // Handle resize events.
  window.addEventListener('resize', () => {
    panel.update();
  });

  let selector = '.jp-ConsolePanel';
  let category = 'Console';
  let command: string;

  // Add the commands.

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

    hboxPanel.spacing = 0;
    dockPanel.spacing = 5;
    hsplitPanel.spacing = 1;

    hboxPanel.direction = 'left-to-right';
    hsplitPanel.orientation = 'horizontal';
    bottomPanel.direction = 'bottom-to-top';

    SplitPanel.setStretch(leftHandler.stackedPanel, 0);
    SplitPanel.setStretch(dockPanel, 1);
    SplitPanel.setStretch(rightHandler.stackedPanel, 0);

    BoxPanel.setStretch(leftHandler.sideBar, 0);
    BoxPanel.setStretch(hsplitPanel, 1);
    BoxPanel.setStretch(rightHandler.sideBar, 0);

    hsplitPanel.addWidget(leftHandler.stackedPanel);
    hsplitPanel.addWidget(dockPanel);
    hsplitPanel.addWidget(rightHandler.stackedPanel);

    hboxPanel.addWidget(leftHandler.sideBar);
    hboxPanel.addWidget(hsplitPanel);
    hboxPanel.addWidget(rightHandler.sideBar);

    rootLayout.direction = 'top-to-bottom';
    rootLayout.spacing = 0; // TODO make this configurable?
    // Use relative sizing to set the width of the side panels.

void docManager
        .newUntitled({
          type: 'file',
          path: fbModel.path
        })
        .then(model => {
          docManager.open(model.path);
        });
    }
  });
  fbWidget.toolbar.insertItem(0, 'create', creator);

  let panel = new SplitPanel();
  panel.id = 'main';
  panel.addWidget(fbWidget);
  SplitPanel.setStretch(fbWidget, 0);
  let dock = new DockPanel();
  panel.addWidget(dock);
  SplitPanel.setStretch(dock, 1);
  dock.spacing = 8;

  document.addEventListener('focus', event => {
    for (let i = 0; i < widgets.length; i++) {
      let widget = widgets[i];
      if (widget.node.contains(event.target as HTMLElement)) {
        activeWidget = widget;
        break;
      }
    }
  });

  // Add commands.

path: fbModel.path
        })
        .then(model => {
          docManager.open(model.path);
        });
    }
  });
  fbWidget.toolbar.insertItem(0, 'create', creator);

  let panel = new SplitPanel();
  panel.id = 'main';
  panel.addWidget(fbWidget);
  SplitPanel.setStretch(fbWidget, 0);
  let dock = new DockPanel();
  panel.addWidget(dock);
  SplitPanel.setStretch(dock, 1);
  dock.spacing = 8;

  document.addEventListener('focus', event => {
    for (let i = 0; i < widgets.length; i++) {
      let widget = widgets[i];
      if (widget.node.contains(event.target as HTMLElement)) {
        activeWidget = widget;
        break;
      }
    }
  });

  // Add commands.
  commands.addCommand('file-open', {
    label: 'Open',
    icon: 'fa fa-folder-open-o',

let consolePanel = new ConsolePanel({
    rendermime,
    manager,
    path,
    contentFactory,
    mimeTypeService: editorServices.mimeTypeService
  });
  consolePanel.title.label = TITLE;

  let palette = new CommandPalette({ commands });

  let panel = new SplitPanel();
  panel.id = 'main';
  panel.orientation = 'horizontal';
  panel.spacing = 0;
  SplitPanel.setStretch(palette, 0);
  SplitPanel.setStretch(consolePanel, 1);
  panel.addWidget(palette);
  panel.addWidget(consolePanel);

  // Attach the panel to the DOM.
  Widget.attach(panel, document.body);

  // Handle resize events.
  window.addEventListener('resize', () => {
    panel.update();
  });

  let selector = '.jp-ConsolePanel';
  let category = 'Console';
  let command: string;

leftHandler.stackedPanel.id = 'jp-left-stack';

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

    hboxPanel.spacing = 0;
    dockPanel.spacing = 5;
    hsplitPanel.spacing = 1;

    hboxPanel.direction = 'left-to-right';
    hsplitPanel.orientation = 'horizontal';
    bottomPanel.direction = 'bottom-to-top';

    SplitPanel.setStretch(leftHandler.stackedPanel, 0);
    SplitPanel.setStretch(dockPanel, 1);
    SplitPanel.setStretch(rightHandler.stackedPanel, 0);

    BoxPanel.setStretch(leftHandler.sideBar, 0);
    BoxPanel.setStretch(hsplitPanel, 1);
    BoxPanel.setStretch(rightHandler.sideBar, 0);

    hsplitPanel.addWidget(leftHandler.stackedPanel);
    hsplitPanel.addWidget(dockPanel);
    hsplitPanel.addWidget(rightHandler.stackedPanel);

    hboxPanel.addWidget(leftHandler.sideBar);
    hboxPanel.addWidget(hsplitPanel);
    hboxPanel.addWidget(rightHandler.sideBar);

    rootLayout.direction = 'top-to-bottom';
    rootLayout.spacing = 0; // TODO make this configurable?