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LoopBack 4 May 2020 Milestone Update

· 7 min read
Janny Hou

Originally published on

The completion of the migration epic would be the biggest news in May. Now LoopBack 3 users can find the migration guide here. Meanwhile, we have feature contributions and bug fixes happened across all the functional areas.

There are more than 20 community PRs merged in May and we really appreciate every community member's help. We set up community calls every four weeks to keep in touch with our maintainers. See the latest schedule and recording in this story.

Keep reading to learn about what happened in May.

Migration Epic

Migrating components

The migration guide for components, which is a powerful way to contribute any artifacts, is the last but most widely covered story in the migration epic. To make the migration guide easier to navigate, we split component-related instructions into several sub-sections as "project layout", "models, entities and repositories", "current context", "model mixins", "REST API endpoints". You can check the documentation migration-extensions-overview and its sub-pages to learn the details.

Migrating LoopBack 3 tests to LoopBack 4

When a LoopBack 3 application is mounted in a LoopBack 4 project, its endpoints are exposed through the LoopBack 4's REST server. To reuse the existing LoopBack 3 tests, you can easily migrate them by following the instructions in example lb3-application. It covers how to set up clients to test requests and how to test runtime functions.


Preserving prototype for toObject

LoopBack CRUD operations invoke toObject function internally to return a model instance. toObject converts a value to a plain object as DTO (Data transfer object). It returned a JSON representation before, which doesn't preserve the prototype of complicated types like Date, ObjectId but returned the value as a string instead. Now such values' prototypes are kept, for example:

const DATE = new Date('2020-05-01');
const created = await repo.create({
createdAt: DATE,
// The returned model instance has `createdAt` as type Date
id: 1,
createdAt: DATE,

Express Middleware

LookBack 4 leverages Express behind the scenes for its REST server implementation. We decided to not use Express middleware as-is but now we support integrating the middleware in different ways. You can invoke it explicitly in the sequence, or register it to be executed by InvokeMiddleware action, or use it as controller interceptors.

Page Express middlware explains all the scenarios and usages. And page Middleware provides the general knowledge of LoopBack 4 middleware.

Context Improvements

  • Function createBindingFromClass allow bindings to be created from dynamic value provider classes, for example:

    @bind({tags: {greeting: 'c'}})
    class DynamicGreetingProvider {
    static value(@inject('currentUser') user: string) {
    return `Hello, ${this.user}`;
    // toDynamicValue() is used internally
    // A tag `{type: 'dynamicValueProvider'}` is added
    const binding = createBindingFromClass(GreetingProvider);
  • A provider class can use dependency injection to receive resolution-related metadata such as context and binding. But the overhead to wrap a factory function is not desired for some use cases. PR#5370 introduces a lightweight alternative using toDynamicValue as follows:

    import {ValueFactory} from '@loopback/context';
    // The factory function now have access extra metadata about the resolution
    const factory: ValueFactory<string> = resolutionCtx => {
    return `Hello, ${}#${
    } ${resolutionCtx.options.session?.getBindingPath()}`;
    const b = ctx.bind('msg').toDynamicValue(factory);

    A benchmark is added to measure the performance of different styles of context bindings in package @loopback/benchmark. You can run npm run -s benchmark:context to see the result.

  • PR#5378 introduced a model booter to automatically bind model classes to the application during boot. You can retrieve and inject model constructors using key models.<model_name>. For example:

    class MyModel extends Model {}

    class MyModelComponent {
    models = [MyModel];
    // you can get MyModel by `models.MyModel`
    const modelCtor = myApp.getSync<typeof MyModel>('models.MyModel');

Build Improvements

  • We upgraded the dependency to TypeScript@3.9.2. Code adjustments including null check and type intersection were made to be compatible with the new version. You can check PR#5041 for more details.

  • Replace eslint rule no-invalid-this with TypeScript-aware one: In code accessing this variable, eslint-ignore comment for no-invalid-this will no longer work. You can either change those comments to disable @typescript-eslint/no-invalid-this, or better tell TypeScript what is the type of this in your function.

    A TypeScript example:

    import {Suite} from 'mocha';
    describe('my mocha suite', function(this: Suite) {
    it('is slow', function(this: Mocha.Context) {

    A JavaScript example:

    describe('my mocha suite', /** @this {Mocha.Suite} */ function() {
    it('is slow', /** @this {Mocha.Context} */ function() {
  • Remove hand-written index files: We removed the root level dummy index files and changed the entry point of project to be the index file inside src folder. An example of the latest layout of a package can be found in the Todo application.

Application Booter

  • You can register a booter to boot a sub-application as:

    class MainAppWithSubAppBooter extends BootMixin(Application) {
    constructor() {
    this.projectRoot = __dirname;
    // boot a sub-application `app`, its bindings will be added as well

Documentation and Blog

What LoopBack can offer on top of Express

LoopBack is a framework built on top of Express. It comes packed with tools, features, and capabilities that enables rapid API and micro-services development and easy maintenance. Last month we published a blog summarizing the points that make LoopBack a compelling choice for Express developers when it comes to API development. You can read this blog to see how LoopBack can bring Express to the next level.

Managing LoopBack APIs with IBM APIConnect

LoopBack 4 application can integrate with API Connect framework. We've prepared an article on how you can take the APIs created from LoopBack and import them into API Connect for API management. Stay tuned for the published article.

Setting Debug String

Documentation setting debug string explains the usage of running a LoopBack 4 application with debug string turned on. You can check the documentation above to learn the debug string pattern and the format in each package.

Strong Error Handler

As a dependency of @loopback/rest, package strong-error-handler is an error handler for use in both development (debug) and production environments. You can use it to customize the error rejection in the LoopBack 4 sequence. For its detailed usage, please read the documentation using string error handler.

Postgresql Connector

We've been sharing the connector documentation with LB3, which might be confusing, especially for new LB4 users. We updated the PostgreSQL connector page and also the tutorial. By walking you through the steps of creating a LB4 application and connecting to the PostgreSQL database, we hope the new tutorial helps new users to pick up LoopBack 4 better.

You can read page connecting to PostgreSQL to follow the tutorial.

Youtube Videos

For creating tutorials, we have more materials than documentations. Last month, one of our core maintainers Miroslav published two video tutorials on our StrongLoop YouTube channel:


There are several documentation and user experience improvements happened this month to make the authentication system more automatic and easy to use:

  • Added example @loopback/todo-jwt to demo enabling JWT authentication in the Todo application. Its corresponding tutorial JWT authentication tutorial is coming soon.

  • Added security specification enhancer in @loopback/authentication-jwt to automatically bind the JWT scheme and global security specification to application. You don't need to manually add them in the application constructor anymore. The updated usage is documented in the file.

June Milestones

This month, we would like to work on the remaining items for the migration guide epic, documentation improvement and more. For more details, take a look at our June milestone list on GitHub.

Call to Action

In 2020, we look forward to helping you and seeing you around! LoopBack's success depends on you. We appreciate your continuous support and engagement to make LoopBack even better and meaningful for your API creation experience. Here's how you can join us and help the project: