Configuration 'production' could not be found in project 'my-lib'

I am building a library with Angular 6.1.0

ng new lib-demo
ng generate library my-lib

All the articles advise running the build for the library with a --prod flag like so:

ng build my-lib --prod

However, this throws an error

Configuration 'production' could not be found in project 'my-lib'.

Which is probably correct because when I look at the angular.json there is no definition for a production build configuration in the library project. It is present for the application project only.

the following is what I have under the build configuration for library project that uses ng-packagr

"build": {
      "builder": "@angular-devkit/build-ng-packagr:build",
      "options": {
        "tsConfig": "projects/my-lib/tsconfig.lib.json",
        "project": "projects/my-lib/ng-package.json"
      }
    }

So the question here is that is the --prod flag not required anymore and just running ng build m-lib will generate a prod build?

Looking at the contents of dist folder it looks so but I am not 100% sure. If someone could validate this, it will be great.

Solution

Beginning with version 6.1, Angular always does a production build of our library, i.e. in new versions of Angular we don't need the --prod flag anymore when building it, libraries are always built in AOT mode. To ensure, you can take a look at these issues in Angular-CLI repository:

https://github.com/angular/angular-cli/issues/12290

https://github.com/angular/angular-cli/issues/12226

And this article ("Building the Library" section):

https://blog.angularindepth.com/creating-a-library-in-angular-6-87799552e7e5

If you are still using version 6.0.x (or lower) you will want to use the --prod flag when building your library.

You still have an option to pass a configuration as a parameter if needed: ng build --configuration=configuration (see docs). If necessary, you can specify build rules in angular.json, for example, for production build:

"configurations": {
    "production": {
        // Options here
    }
}

And the command should be ng build --configuration=production.