App triggers prerendering when `prerender` property is set to `false` in `angular.json`

[pawelfras]

Command

build

Is this a regression?

• Yes, this behavior used to work in the previous version

The previous version in which this bug was not present was

19.0.0-next.9

Description

OOTB Angular 19.0.0-next.13 app triggers prerendering when prerender property is set to false in angular.json as well as when parameter --prerender=false is passed to the build command.

Prerendering is triggered even after removing app.server.routes.ts which by default sets renderMode: RenderMode.Prerender for routes.

Minimal Reproduction

• create new Angular 19.0.0-next.13 app with SSR/SSG
• in OOTB app, inside angular.json, add "prerender": false to architecture -> build -> options object
• run npm run build
• in console, verify if any static route is prerendered
  
• in dist/<your_app> location, verify if prerendered-routes.json is generated
• discard changes from the angular.json file
• run npm run build -- --prerender=false
• follow previous steps to verify that perrendering was triggered

Exception or Error

No response

Your Environment

Angular CLI: 19.0.0-next.13
Node: 22.8.0
Package Manager: npm 10.8.2
OS: darwin arm64

Angular: 19.0.0-next.11
... animations, common, compiler, compiler-cli, core, forms
... platform-browser, platform-browser-dynamic, platform-server
... router

Package Version

@angular-devkit/architect 0.1900.0-next.13
@angular-devkit/build-angular 19.0.0-next.13
@angular-devkit/core 19.0.0-next.13
@angular-devkit/schematics 19.0.0-next.13
@angular/cli 19.0.0-next.13
@angular/ssr 19.0.0-next.13
@schematics/angular 19.0.0-next.13
rxjs 7.8.1
typescript 5.6.3
zone.js 0.15.0

Anything else relevant?

No response

[alan-agius4]

When the outputMode option is specified, both the prerender and appShell options become noop. A warning should be issued to indicate this.

To fully disable server-side building and generation, including disabling server bundle generation and prerendering, the following command will be made available:

ng b --output-mode=static --no-server

I’ll open a PR to address both issues.

[pawelfras]

Hi @alan-agius4,
Thank you very much for your work, and much appreciated for providing a new build option!
Do I understand correctly that --no-server will disable SSR completely? I'm wondering if there is a chance to easily disable the SSG itself while still allowing server-side rendering.

[alan-agius4]

Do I understand correctly that --no-server will disable SSR completely?

Yes, that is correct.

I'm wondering if there is a chance to easily disable the SSG itself while still allowing server-side rendering.

The SSR/SSG settings will be influenced by the routing configuration. What is your use case for disabling SSG while still allowing SSR, particularly if one or more routes are explicitly configured for SSG?

[pawelfras]

Thank you for your answer!

The SSR/SSG settings will be influenced by the routing configuration. What is your use case for disabling SSG while still allowing SSR, particularly if one or more routes are explicitly configured for SSG?

I'd like to include SSR in an app where SSG is not configured for any router. I apologise if I did not make myself clear.

In previous versions (19.0.0-next.9 and below) where was an old API in server.ts no server routing configuration (apologies, I'm not sure whether it's relevant information) it could be seamlessly achieved by setting prerender option to false in angular.json or passing it to ng build command.

Btw. I double-checked the app behaviour and now, for reasons unknown for me, prerendering occurs even after disabling routing configuration:

[alan-agius4]

With the new API, you'd need to update the app.routes.server.ts and configure all routes to be SSR.

import { RenderMode, ServerRoute } from '@angular/ssr';

export const serverRoutes: ServerRoute[] = [
  {
    path: '**',
-   renderMode: RenderMode.Prerender
+   renderMode: RenderMode.Server
  }
];

In this case, because there are no routes configured, app.component is treated as the root page.

[pawelfras]

In the scenario above I detached server routing config compeltely from server app config.
Does it mean RenderMode.Prerender is a default value even if no ServerRoute[] config is provided?

EDIT: Got an answer here:

In this case, because there are no routes configured, app.component is treated as the root page.

Thanks, understood!

With the new API, you'd need to update the app.routes.server.ts and configure all routes to be SSR.

Is this the only place that requires adjustment? I quickly checked it and for some reasons prerendering still takes place.

[alan-agius4]

So when the Angular router, does not have any routes we do fallback to prerendering the index.html.

[pawelfras]

Thank you very much for the explanation!

One thing came to my mind (no issue spotted, sharing this just as a food for thoughts) - let's assume, that for some business reasons, I changed index property in angular.json to the following:

- "index": "src/index.html",
+ "index": {
+    "input": "src/index.html",
+    "output": "index.html"
+},

So now my index file in browser/ directory is index.html. In case of fallback you mentioned, index.html will be overwritten by the prerendered template.
As the fallback applies only if there are no configured routes, probably there won't be an issue that some CSR page will use prerendered template (which might lead to flickering).

Thanks again for your help!

[pawelfras]

So when the Angular router, does not have any routes we do fallback to prerendering the index.html.

Apologies, few more questions:

• Could this fallback behaviour highlighted in docs? It might not that obvious especially that before prerendering could be opted out and now it can be achieved as a side effect only when routes are configured (and are not set to RenderMode.Prerender)
• is this statement true also for the old API (no outputMode, no server routing, no new API in server.ts)?

I will repeat myself, but once again thank you @alan-agius4 for your work and openness to discussion. Cheers!