How do I add Sass compilation in Angular CLI 6: angular.json?

Currently on version 9.0.6 this is a little different. Per Dico and Smokey Dawson above you would use the following in your angular.json:

"projects": {
    "angular-app": {
        "root": "",
        "sourceRoot": "src",
        "projectType": "application",
        "prefix": "app",
        "schematics": {
            "@schematics/angular:component": {
                "styleext": "scss"
            }
        },
        "architect": {...}
    }
}

But in later versions the styleext property has changed to just style. Sourced from Angular Github here.

Doing exactly what Smokey Dawson commented,

"projects": {
  "angular-app": {
    "root": "",
    "sourceRoot": "src",
    "projectType": "application",
    "prefix": "app",
    "schematics": {
      "@schematics/angular:component": {
        "styleext": "scss"
      }
    },
    "architect": {...}
  }
}

The above is the result. So in your app, under the key schematics, add a key @schematics/angular:component with the key styleext set to scss.

This should be applied to your angular.json file in the root directory of your project.

In Angular 6, It's more easier then that. The compilation is handled automatically! How that ? To use scss at components level you need to name the file with the extension scss (same go for less, styl). And in your component.ts you change the corresponding style to scss too.

Here a screenshoot from the doc:

And to use Scss at a globale scale (src/styles.css), then you need to change styles.css to styles.scss (same go for less, styl ...etc). And then you go on angular.json and change the old styles.css to the new styles.scss value. As shown in the image bellow:

Here the link for the doc for the first part for the ones that want to explore themselves: https://angular.io/guide/component-styles#non-css-style-files

Now what about setting up, ng-cli so when generating new component the extension will be by default scss ? Use the ng config command as follow:

 ng config schematics.@schematics/angular:component.styleext scss

That will update angular.json with:

"schematics": {
    "@schematics/angular:component": {
        "styleext": "scss"
    }
} 

Which is the equivalent to the old angular-cli.json

    defaults: {
        "styleext": "scss"
    }

That's for a project that is already started but not set to default to scss. If you are creating a new project then use the option --style to specify that as follow:

ng new my-project --style=scss

And you will not have to use the precedent command which is a config command that allow us to update the specified field in the angular.json file (because it will be already set).