@loopback/docs

--- lang: en title: 'Frequently-asked questions' keywords: LoopBack 4.0, LoopBack 4, Node.js, TypeScript, OpenAPI sidebar: lb4_sidebar permalink: /doc/en/lb4/FAQ.html summary: LoopBack 4 is a completely new framework, also known as LoopBack-Next. --- ## About the project ### What’s the vision behind LoopBack 4? - Make it even easier to build apps that require complex integrations - Enabling an ecosystem of extensions - Small, fast, flexible, powerful core - Suitable for small and large teams - Minimally opinionated, enforce your team's opinions instead See [Crafting LoopBack 4](Crafting-LoopBack-4.md) for more details. ### Is LoopBack 4 a REST framework? **Yes and no.** Unlike other REST frameworks, LoopBack 4's scope spans beyond creating REST APIs, and the individual Node.js packages are designed with this in mind. This leads to increased flexibility for more complex use-cases and enables reuse of concepts across different projects. LoopBack 4's focus is still REST APIs and will continue to innovate in improving the API developer experience. See [Can non-REST projects utilize LoopBack 4?](#can-non-rest-projects-utilize-loopback-4) for details on how to leverage LoopBack in other projects. ### Why introduce so many new concepts? At first glance, concepts such as Inversion of Control, Dependency Injection, Services and Providers may seem daunting when coming from other typical Node.js projects. However, these have been well-defined and battle-tested in many other programming languages such as C# and Java. Furthermore, they have been rapidly adopted by other Node.js project such as Angular. These concepts improve code readability and predictability by promoting use of SOLID principles, and by doing so, aid in code maintainability in the long term. ### Can non-REST projects utilize LoopBack 4? **Yes they can!** LoopBack 4 was designed to be flexible to support non-REST projects. Here's a few examples: - `@loopback/context` Dependency injection framework. Can be used to in any Node.js project to improve code maintainability. - `@loopback/core` Builds on top of `@loopback/context` and introduces higher-level concepts such as Services to further improve code maintainability. - `@loopback/metadata` Create and inspect TypeScript decorators to define, merge and inspect metadata. - `@loopback/repository` Cross-database ORM and ODM framework. Can be used to create cross-database designs, queries and relations. - `@loopback/filter` Contains LoopBack filter typings and builders. Can be used to build queries on LoopBack API consumers. - `@loopback/graphql` Brings first-class support for building GraphQL APIs. Can be used alongside or independent of `@loopback/rest`. ### Can I still use `require` and `import` without Dependency Injection? **Yes you can!** LoopBack 4 projects are still Node.js projects at its core. Hence, developers still can leverage on existing and familiar Node.js concepts to build their applications. We still recommend utilizing the Dependency Injection framework where possible to improve code maintainability. ### LoopBack 3 vs LoopBack 4 We recommend that current users migrate to LoopBack 4, which can be done through following the [migration guide](migration/overview.md) and new users start with LoopBack 4. See [Differences between LoopBack v3 and v4](Understanding-the-differences.md) if you're interested in the differences between the two versions. ### Why TypeScript? Although developers can still write application logic in either JavaScript or TypeScript, LoopBack 4's core is written in TypeScript, for the following reasons: - **Improved developer productivity and scalability**. Our customers need a framework that scales to dozens and even hundreds of developers. This scalability is the reason TypeScript exists and is gaining traction. - **Improved extensibility** and flexibility. LoopBack 4's core is simpler than LoopBack 3.x with well-defined extension points. A lot of responsibility will be shifted to extensions (components), which can be JavaScript or TypeScript. - **Unified tooling**. A good proportion of TypeScript developers use Visual Studio Code. Hence, we provide out-of-the-box Visual Studio Code configurations to make LoopBack application development more convenient. - **Future-proofing**. Ability to leverage the latest and future JavaScript constructs. TypeScript's support for static analysis makes more robust tooling possible and is the foundation for its scalability. The ability to easily refactor code without the common human-introduced errors. Dev and Compile time checking. For example, most people don't have the same expertise and time we do to setup complex linting solutions (for example, a linting config that works across many projects). For more details, see the lengthy discussion in [#6](https://github.com/loopbackio/loopback-next/issues/6). ## About the documentation ### Where are the tutorials? See [Examples](Examples.md) and [Tutorials](Tutorials.md). ### What features are planned? Our roadmap is captured in [here](https://github.com/loopbackio/loopback-next/labels/roadmap). You can also check out our [monthly milestone plans](https://github.com/loopbackio/loopback-next/labels/Monthly%20Milestone). For LoopBack 3 users, here is the list of LB3 feature parity we wish to be implemented in the future: https://github.com/loopbackio/loopback-next/issues/1920. Do you have some other features in mind? Add your feature requests at [loopback-next/issues/new](https://github.com/loopbackio/loopback-next/issues/new). ## Technical usage ### Does JavaScript still work? **No.** While some packages _may_ work with JavaScript, there is no guarantee that all features will be compatible. Please see GitHub issues [#560](https://github.com/loopbackio/loopback-next/issues/560) and [#1978](https://github.com/loopbackio/loopback-next/issues/1978) for prior discussion and the issues of supporting a pure-JavaScript experience. ### How do I disable the API Explorer? LoopBack provides a self-hosted and a redirect to an [external API Explorer](https://explorer.loopback.io). Documentation to disable both API Explorers: - [Disable redirect to API Explorer](https://loopback.io/doc/en/lb4/Server.html#disable-redirect-to-api-explorer) - [Disable Self-Hosted API Explorer](https://loopback.io/doc/en/lb4/Self-hosted-rest-api-explorer.html#disable-self-hosted-api-explorer) ### How do I send a custom response? #### Return a custom response body The response is usually determined by value returned by the controller function that's invoked. A good example of this is the default `PingController` scaffolded with every LoopBack 4 application: ```ts // Note: code shortened for bevity // Map to `GET /ping` @get('/ping', { responses: { '200': PING_RESPONSE, }, }) ping(): object { // Reply with a greeting, the current time, the url, and request headers return { greeting: 'Hello from LoopBack', date: new Date(), url: this.req.url, headers: Object.assign({}, this.req.headers), }; } } ``` In this example, we can see that the `ping()` function returns a custom object. This would be reflected in the response body when an API consumer makes a request to `/ping`. #### Further customize the response Sometimes, other parts of the response needs to be modified (such as the HTTP headers). This can be accomplished by injecting the `Response` object into the controller: ```ts import {inject} from '@loopback/core'; import {get, Response, RestBindings} from '@loopback/rest'; export class PingController { constructor(@inject(RestBindings.Http.RESPONSE) private res: Response) {} // Map to `GET /ping` @get('/ping', { responses: { '200': { description: 'Ping Response', content: { 'application/json': { schema: { type: 'object', title: 'PingResponse', properties: { greeting: {type: 'string'}, additionalProperties: false, }, }, }, }, }, }, }) ping(): object { this.res.setHeader('x-secret-sauce', 'Sugar, spice and everything nice.'); // Reply with a greeting return { greeting: 'Hello from LoopBack', }; } } ``` This will result in a custom response body and a new header, `x-secret-sauce`. {% include note.html content="While LoopBack 4 currently doesn't validate the server response against the OpenAPI Spec, it's a good idea to keep the `responses` object in-line with the actual responses (as shown in the code above) to prevent API consumers and OAS 3 generators from being misled into expecting a different response." %} ### Where do I find the default binding keys? Binding keys used by `@loopback/*` packages are consolidated under [Reserved binding keys](reference/reserved-binding-keys.md). ### What is the difference between general and configuration bindings? Configuration bindings leverage an existing binding key to transparently standardize the naming convention of the configuration binding keys without needing to create a completely separate key. See [Configuration by convention](https://loopback.io/doc/en/lb4/Context.html#configuration-by-convention). ### Can I attach an Express router? Yes. See [Mounting an Express Router](Route.md#mounting-an-express-router). ### Can I mount an Express middleware? Yes. See [Using Express Middleware](Express-middleware.md). ### LoopBack middleware vs Express middleware Where possible, use LoopBack middleware instead. Express middleware are less flexible and LoopBack integration is only meant to be a compatibility layer for **existing** middleware. See [Middleware](Middleware.md) for more details.