Frontend Developer Guide#

This document aims to describe the current LIT frontend system, including conventions, best practices, and gotchas.

High Level Overview#

LIT is powered by two central pieces of tech - lit-element for components and HTML rendering, and mobx for observable-oriented state management.

Lit-element is a simple, web-component based library for building small, self-contained pieces of web functionality. It uses a template-string based output to declaratively render small, isolated pieces of UI.

Mobx is a tool centered around observable data, and it makes managing state simple and scalable.

We highly recommend reading the docs for both projects - they both have fairly simple APIs and are easy to digest in comparison to some heavier-weight toolkits like Angular.

Application Architecture#

The LIT client frontend is roughly divided into three conceptual groups - Modules (which render visualizations), Services (which manage data), and the App itself (which coordinates initialization of services and determines which modules to render).


The LIT app bootstrapping takes place in two steps: First, the served index.html page contains a single web component for the <lit-app>. This component is responsible for the overall layout of the app, including the toolbar, footer, and the <lit-modules> component. The <lit-modules> component is responsible for actually laying out and rendering the various LitModule components, a process about which we’ll go into greater detail later.

The JS bundle entry point is main.ts, which first imports the loaded, the <lit-app> web component is declared, and attaches itself to the DOM, waiting for the app to be initialized.

The second step is kicking off app initialization. The LitApp singleton class is provided with a layout declaring which LitModule components to use, then builds the app services and kicks off app initialization and loading data.


A layout defines the arraignment of LitModule classes in the UI. Layouts are specified in Python as LitCanonicalLayout instances, and LIT includes three pre-configured layouts in

  • simple: A minimalist layout with the examples on top (either individually (selected by default) or in a table) and predictions on the bottom;

  • default: The original LIT layout with a single group of modules on top for exploring and selecting data, and a collection of tabs supporting different analytical tasks on the bottom; and

  • experimental: A three-panel layout that puts exploratory data visualizations at full-page height on the left, tools for inspecting and manipulating examples and their associated predictions in the upper right, and a collection of tabs supporting different analytical tasks in the lower left. Note that this was introduced in v1.0 as an experimental feature, your feedback is appreciated.

You can also add custom layouts to your LIT instance by defining one or more LitCanonicalLayout instances and passing them to the server. For an example, see CUSTOM_LAYOUTS in

Note: The pre-configured layouts are added to every LitApp instance using dictionary updates where the Mapping passed to the LitApp constructor overrides the pre-configured layouts Mapping. Thus, you can remove or change these pre-configured layouts as you like by passing a Mapping where the values of simple, default, and/or experimental is None (to remove) or a LitCanonicalLayout instance (to override) as you desire.

The actual layout of components in the LIT UI, see <lit-modules>, can be different than the declared layout, since the visibility of modules depends on a number of factors, including the user-chosen visibility, the compatibility of the configured modules with the selected model and dataset, and whether or not specific modules show multiple copies per selected model. The actual layout is computed in modules_service.


Finally, the LIT App initializes by building the various service classes and starting the initial load of data from the server. This process consists of:

  1. Parsing the URL query params to get the url configuration

  2. Fetching the app metadata, which includes what models/datasets are available to use.

  3. Determining which models/datasets to load and then loading them.

Modules (LitModule)#

The LitModule is the base class from which all module components derive. It provides a number of convenience methods for handling common update / data loading patterns. Each LIT Module also requires a few static methods by convention, responsible for specifying Module display and behavior. These helpers and conventions are outlined below:

 * A dummy module that responds to changes in selected data by making a request
 * to an API service to get the pig latin translation.
@customElement('demo-module')                                                   // (0)
export class DemoTextModule extends LitModule {
  static override title = 'Demo Module';                                        // (1)
  static override template =
      (model: string, selectionServiceIndex: number, shouldReact: number) =>    // (2)
          html`<demo-module model=${model} .shouldReact=${shouldReact}
  static override duplicateForModelComparison = true;                           // (3)

  static override get styles() {
    return [styles];                                                            // (4)

  private readonly colorService = app.getService(ColorService);                 // (5)

  @observable private pigLatin: string = '';                                    // (6)

  override firstUpdated() {
    this.reactImmediately(() => this.selectionService.primarySelectedInputData, // (7)
      primarySelectedInputData => {

  private async getTranslation(primarySelectedInputData: IndexedInput) {
    if (primarySelectedInputData === null) return;

    const promise = this.apiService.getPigLatin(primarySelectedInputData);      // (8)
    const results = await this.loadLatest('pigLatin', promise);                 // (9)
    if (results === null) return;

    this.pigLatin = results;

  override renderImpl() {                                                       // (10)
    const color = this.colorService.getDatapointColor(
    return html`
      <div class="results" style=${styleMap({'color': color})}>

  static checkModule(modelSpecs: ModelsMap, datasetSpec: Spec): boolean {       // (11)
    return true;

declare global {                                                                // (12)
  interface HTMLElementTagNameMap {
    'demo-module': DemoTextModule;

The above LitModule, while just a dummy example, illustrates all of the necessary static properties and many of the most common patterns found in the LIT app.


First, a LitModule must declare a static title string (1) and template function (2). The template function determines how the modules layout renders the component template and passes in module properties, such as the name of the model this should respond to. (3) specified behavior in model comparison mode; if duplicate is set to true, the layout engine will create two (or more) instances of this module, each responsible for a different model.

Note: there are additional static attributes which control module behavior; see the LitModule base class for full definitions.

Styles are also declared with a static get method (4), following the lit-element convention. These styles can be built using the lit-element css template function, or by importing a separate .css file. Styles can be shared between components by importing a shared styles .css file (for instance, shared_styles.css)

Services are used by requesting them from the LitApp app singleton class (5). This can be thought of as a super-simple dependency injection system, and allows for much easier stubbing / mocking of services in testing. We request the colorService here, but the base LitModule class initializes the most common services (apiService, appState, and selectionService) for us automatically.

The LitModule must also provide a static checkModule (11) method, which determines if this module should display for the given model(s) and dataset.

Finally, the @customElement('demo-module') decorator (0) defines this class as a custom HTML element <demo-module>, and (12) ensures this is accessible to other TypeScript files in different build units.


The above module has a very simple task - When the user selects input data, it makes a request to an API service to fetch and display a pig latin translation of the data. Since we’re using mobx observables to store and compute our state, we do this all in a reactive way.

First, since the LitModule base class derives from MobxLitElement, any observable data that we use in the renderImpl method automatically triggers a re-render when updated. This is excellent for simple use cases, but what about when we want to trigger more complex behavior, such as the asynchronous request outlined above?

The pattern that we leverage across the app is as follows: The renderImpl method (10) accesses a private observable pigLatin property (6) that, when updated, will re-render the template and show the results of the translation automatically. In order to update the pigLatin observable, we need to set up a bit of machinery. In the lit-element lifecycle method firstUpdated, we use a helper method reactImmediately (7) to set up an explicit reaction to the user selecting data. Whatever is returned by the first function (in this case this.selectionService.primarySelectedInputData) is observed and passed to the second function immediately and whenever it changes, allowing us to do something whenever the selection changes. Note, another helper method react is used in the same way as reactImmediately, in instances where you don’t want to immediately invoke the reaction. Also note that modules should override renderImpl and not the base render method as our LitModule base class overrides render with custom logic which calls our renderImpl method for modules to perform their rendering in.

We pass the selection to the getTranslation method to fetch the data from our API service. However rather than awaiting our API request directly, we pass the request promise (8) to another helper method loadLatest (9). This ensures that we won’t have any race conditions if, for instance, the user selects different data rapidly - the function returns null when the request being fetched has been superseded by a more recent call to the same endpoint. Finally, we set the private pigLatin observable with the results of our API request and the template is automatically rerendered, displaying our data.

This may seem like a bit of work for a simple module, but the pattern of using purely observable data to declaratively specify what gets rendered is very powerful for simplifying the logic around building larger, more complex components.

Escape Hatches#

Finally, it’s worth noting that the declarative template-based rendering setup, while effective for handling most component render logic, is sometimes inadequate for more advanced visualizations. In particular, the template approach is not well suited for animations, rapidly changing data, or things that MUST be done imperatively (such as drawing to a canvas). Fortunately, it’s very easy to “bridge” from declarative to imperative code by leveraging the lit-element lifecycle methods.

In particular, the updated and firstUpdated methods are useful for explicitly doing work after the component has rendered. You can use normal querySelector methods to select elements and update their properties imperatively (note that you must make selections using the shadow root, not the document, since we’re using isolated web components).

One important caveat is that messing with the actual structure of the rendered DOM output (such as removing/reordering DOM nodes) will cause issues with lit-element, since it relies on a consistent template output to do its reconciliation of what needs to be updated per render.

// An example of a LITModule imperative "escape hatch"
  updated() {
    const canvas = this.shadowRoot!.querySelector('canvas');

  override renderImpl() {
    return html`<canvas></canvas>`;

Stateful Child Elements#

Some modules may contain stateful child elements, where the element has some internal state that can have an effect on the module that contains it. Examples of this include any modules that contain the core/faceting_control.ts element.

With these types of child elements, it’s important for the containing module to construct them programmatically and store them in a class member variable, as opposed to only constructing them in the module’s html template string returned by the renderImpl method. Otherwise they will be destroyed and recreated when a module is hidden off-screen and then brought back on-screen, leading them to lose whatever state they previously held. Below is a snippet of example code to handle these types of elements.

// An example of a LITModule using a stateful child element.
export class ExampleModule extends LitModule {
  private readonly facetingControl = document.createElement('faceting-control');

  constructor() {

    const facetsChange = (event: CustomEvent<FacetsChange>) => {
      // Do something with the information from the event.
    // Set the necessary properties on the faceting-control element.
    this.facetingControl.contextName = ExampleModule.title;
        'facets-change', facetsChange as EventListener)

  override renderImpl() {
    // Render the faceting-control element.
    return html`${this.facetingControl}`;

Style Guide#

  • Please disable clang-format on lit-html templates and format these manually instead:

    // clang-format off
    return html`
      <div class=...>
        <button id=... @click=${doSomething}>Foo</button>
    // clang format on
  • For new modules, in most cases you should implement two classes: one module (subclassing LitModule) that interfaces with the LIT framework, and another element which subclasses LitElement, MobxLitElement, or preferably, ReactiveElement, and implements self-contained visualization code. For an example, see modules/annotated_text_module.ts and elements/annotated_text_vis.ts.

  • On supported components (ReactiveElement and LitModule), use this.react() or this.reactImmediately() instead of registering reactions directly. This ensures that reactions will be properly cleaned up if the element is later removed (such as a layout change or leaving comparison mode).

  • Use shared styles when possible.

Development Tips (open-source)#

If you’re modifying any TypeScript code, you’ll need to re-build the frontend. You can have yarn do this automatically. In one terminal, run:

cd ~/lit/lit_nlp
yarn build --watch

And in the second, run the LIT server:

cd ~/lit
python -m lit_nlp.examples.<example_name> --port=5432 [optional --args]

You can then access the LIT UI at http://localhost:5432.

If you only change frontend files, you can use Ctrl/Cmd+Shift+R to do a hard refresh in your browser, and it should automatically pick up the updated source from the build output.

If you’re modifying the Python backend, there is experimental support for hot-reloading the LIT application logic ( and some dependencies without needing to reload models or datasets. See for details.

You can use the --data_dir flag (see to save the predictions cache to disk, and automatically reload it on a subsequent run. In conjunction with --warm_start, you can use this to avoid re-running inference during development - though if you modify the model at all, you should be sure to remove any stale cache files.

Custom Client / Modules#

The LIT frontend can be extended with custom visualizations or interactive modules, though this is currently provided as “best effort” support and the API is not as mature as for Python extensions.

An example of a custom LIT client application, including a custom (potato-themed) module can be found in lit_nlp/examples/custom_module. You need only define any custom modules (subclass of LitModule) and include them in the build.

When you build the app, specify the directory to build with the flag. For example, to build the custom_module demo app:

yarn build

This builds the client app and moves all static assets to a build directory in the specified directory containing the main.ts file (examples/custom_module/build).

Finally, to serve the bundle, set the client_root flag in your python code to point to this build directory. For this example we specify the build directory in examples/custom_module/

# Here, client build/ is in the same directory as this file
parent_dir = os.path.join(pathlib.Path(__file__).parent.absolute()
FLAGS.set_default("client_root", parent_dir, "build"))

You must also define a custom layout definition in Python which references your new module. Note that because Python enums are not extensible, you need to reference the custom module using its HTML tag name:

modules = layout.LitModuleName
POTATO_LAYOUT = layout.LitCanonicalLayout(
        "Main": [modules.DatapointEditorModule, modules.ClassificationModule],
        "Data": [modules.DataTableModule, "potato-module"],
    description="Custom layout with our spud-tastic potato module.",

See for the full example.