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OpenAPI Specification Enhancer

The APIs in a LoopBack RestApplication are described by the OpenAPI Specification (short for OAS). An application’s OAS is mainly generated from controllers and their members’ metadata. Besides this, we would also like to contribute specifications from other places. Therefore, an extension point OASEnhancerService is created to allow registered extensions to provide their OAS fragments and modify a rest application’s specification.

Read about the extension point/extension pattern in documentation

Adding a New OAS Enhancer

Interface OASEnhancer provided by @loopback/rest is describing the specification enhancers. A typical OAS enhancer class should have a string type name field and a function modifySpec() to modify the current specification.

For example, to modify the info field of an OAS, you can create an InfoSpecEnhancer that implements interface OASEnhancer as follows:

import {injectable} from '@loopback/core';
import {
} from '@loopback/rest';

 * A spec enhancer to add OpenAPI info spec
export class InfoSpecEnhancer implements OASEnhancer {
  // give your enhancer a proper name
  name = 'info';

  // takes in the current spec, modifies it, and returns a new one
  // NOTE: use async function if it contains async operation to the spec
  modifySpec(spec: OpenApiSpec): OpenApiSpec {
    const InfoPatchSpec = {
      info: {title: 'LoopBack Test Application', version: '1.0.1'},
    // the example calls a default helper function to merge the fragment spec.
    const mergedSpec = mergeOpenAPISpec(spec, InfoPatchSpec);
    return mergedSpec;
  • The class is decorated with a binding template asSpecEnhancer.
  • The enhancer has a name as info. Name can be used to retrieve a certain enhancer (explained in the extension point section).
  • The enhancer changes the current specification’s info object in function modifySpec.
  • It calls mergeOpenAPISpec to merge the specification fragment into the current spec.

If you need to asynchronously update the spec, you should define the modifySpec function use keyword async and return a Promise as following:

async modifySpec(spec: OpenApiSpec): Promise<OpenApiSpec> {
  return modifySpecAsync(spec);

Default Merge Function

Since modifySpec has full access to the current spec, it can perform any operation: merge, delete, or more complicated changes. This is totally determined by the extension contributor.

To apply the basic merging, we provide a default helper function called mergeOpenAPISpec that leverages json-merge-patch to merge two json objects. You can find its usage in the previous section

Registering an Enhancer

After decorating your enhancer properly with @injectable(asSpecEnhancer), you can bind it to your application as follows:

import {createBindingFromClass} from '@loopback/core';
import {InfoSpecEnhancer} from './enhancers/infoSpecEnhancer';

class MyApplication extends RestApplication {
  constructor() {

OAS Enhancer Service as Extension Point

The OAS enhancer extension point is provided by package @loopback/rest. It organizes the registered OAS enhancers, and provides APIs to either apply one enhancer by name, or apply all enhancers automatically.

Registering an Enhancer Service

You can bind the OAS enhancer extension point to your app via key OASEnhancerBindings.OAS_ENHANCER_SERVICE:

import {RestApplication} from '@loopback/rest';
import {OASEnhancerService, OASEnhancerBindings} from '@loopback/rest';

class MyApplication extends RestApplication {
  constructor() {
      createBindingFromClass(OASEnhancerService, {
        key: OASEnhancerBindings.OAS_ENHANCER_SERVICE,

  // define a function to return a spec service by the same key
  getSpecService() {
    return this.get(OASEnhancerBindings.OAS_ENHANCER_SERVICE);

Applying Registered Enhancers

To automatically apply all the registered enhancers, call applyAllEnhancers:

await app.getSpecService.applyAllEnhancers();

In the future we will support applying enhancers by a custom sequence. The sequence will be determined by a combination of group names and the alphabetical order.

To retrieve an enhancer by name, call getEnhancerByName:

await app.getSpecService.getEnhancerByName('info');

To apply an enhancer by name, call applyEnhancerByName:

await app.getSpecService.applyEnhancerByName('info');

Usage in REST Server

The REST server in @loopback/rest has a built-in OAS enhancer service. By default it applies all the enhancers bound to its application. You can read section Enhance OpenAPI Specification to learn more about how to add your own enhancers.