perf: added possibility to lazy initialize the document

[H4ad]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Currently, NestJS builds the Swagger document synchronously on startup, which it's a waste of time since we don't care about the swagger routes until we need to make a call to it.

What is the new behavior?

So, I added an option to lazy initialize the Swagger document, the document will only be built when we have some request for it.

In my API, it reduced about 70~75ms from initialization, it will vary according to the API and the amount of routes but now it will only take 2ms to initialize the Swagger Module.

Usage

Now, to use the new lazy loading feature, just change these lines:

const swaggerDocument = new DocumentBuilder()
  .setTitle(config.get<string>('SWAGGER_TITLE') ?? 'Base')
  .setDescription(config.get<string>('SWAGGER_DESCRIPTION') ?? 'The base API')
  .setVersion(config.get<string>('VERSION') ?? '1.0.0')
  .addTag('main')
  .addBearerAuth()
  .build();

-const document = SwaggerModule.createDocument(app, swaggerDocument);
+const document = () => SwaggerModule.createDocument(app, swaggerDocument);

SwaggerModule.setup('/swagger', app, document, {
  swaggerOptions: { docExpansion: 'none' },
});

Just one line to use it.

Reproduction

To use it: https://stackblitz.com/edit/nestjs-starter-bql4af?file=src/main.ts

Does this PR introduce a breaking change?

• Yes
• No

[micalevisk]

A good improvement even for that small app 🎉

eager	lazy

---

I wonder if the API couldn't be this instead:

const lazyDocument = () => SwaggerModule.createDocument(app, swaggerDocument)
SwaggerModule.setup('/swagger', app, lazyDocument, {
  swaggerOptions: { docExpansion: 'none' }
})

[H4ad]

@micalevisk Yeah, I think your way could be better since I don't need to add more options to SwaggerCustomOptions.

[kamilmysliwiec]

@micalevisk suggestions looks good!

Also, @H4ad would you like to create a PR to the docs (describing this feature)?

[H4ad]

Updated with the new version, it was cleaner than the older version.

[H4ad]

Docs PR: nestjs/docs.nestjs.com#2704

[Tony133]

Just some minor corrections for code identation, for the rest is ok 👍

[H4ad]

Fixed issues with e2e tests!

[kamilmysliwiec]

It seems that the PR caused this regression #2481