Building Schematics with a Sandbox

Build Schematics quick and easy with visual diffs.


Testing Schematics

When I started writing schematics I found it challenging to visually see what my code was doing against an Angular application. I wanted a quick feedback loop similar to what front end development has like live-reload. Before each release, I wanted to run the schematic against a real Angular application and assert that the schematic ran successfully while not breaking anything in the process. These testing needs guided me to build a Sandbox for developing schematics. You can find a starter project with it here.

What I described above shares a lot in common with E2E tests. Though, it doesn't come with many of the common shortcomings such as slow execution and brittleness. They're extremely accurate ways to test applications. No simulations, no mocking, no configs. Just the code executing in its targeted environment.

Schematics can also be unit tested just like any other unit of code and provide the benefits that unit tests generally do. They’re fast, isolated, easy to mock, and can provide a quick feedback loop. That being said, this article will focus on the Sandbox method.

The Sandbox

The sandbox is a version controlled directory in your schematic repository that will generally contain an Angular application. For testing and development, you'll execute your schematic against the application in the sandbox. Since the sandbox is version controlled, you can quickly see the changes made to the Angular app after the schematic is run, and have the ability to reset to a known pre-test state as often as you need.

With a few scripts that we'll set up below, you can repeatedly run the schematic during development and easily see a diff of what your schematic does in the sandbox. Since we're scripting this behavior, sandbox testing can be included as part of a test script before releases.

Architecture overview

The sandbox directory will contain a version-controlled Angular application that the schematic will run against through npm/yarn linking.

├── sandbox
│   └── angular-app/
├── src
│   ├── schematic
│   └── collection.json
└── package.json

Version control

Placing the sandbox inside the schematic repository provides several benefits.

  • Everyone develops against the same target app, or apps if you configure more than one.
  • Once you execute the schematic, it’s trivial to see a diff of the changes to your sandbox app in your editor. This is the number one benefit and my initial goal when I started trying to solve this problem.


  • With a short git command that I’ll introduce below, we can easily reset the state of the sandbox and re-run the schematic. This provides a short and fast feedback loop. Make a change, run the schematic, visually and/or programmatically assert, clean the sandbox, and repeat.
  • Since the sandbox is part of the repository, you can tie this into an automated release process where the schematic is tested against the sandbox, assertions are made, and the release is published.

Adding a sandbox

First, add a sibling ./sandbox directory next to your schematic ./src directory. Inside you can create whatever type of application you expect your schematic to run against. I generally run ng new in the sandbox but have also configured an application with more specific files/folders/features to aid in testing.

You may also configure multiple sandboxes, however I will not be covering that here. If that’s something you require, you’ll need to refactor your scripts to run against each sandbox. We’ll cover the scripts below.

Sandbox Preparation

I generally make two changes to the sandbox scripts in the package.json. I use both of these commands when testing the sandbox to ensure we are testing production code, and to avoid issues with watched tests.

"build": "ng build --prod --progress=false",
"test": "ng test --watch=false"

Add scripts

Once you’ve added the sandbox app, the rest of the configuration lies in how you orchestrate the schematic and the sandbox.

For these commands, I’m going to reference yarn, which is similar to npm but a little more concise when running scripts. If you prefer npm, replace the yarn scripts with npm run.

Here are the final scripts you’ll need in the schematics package.json file.

"scripts": {
    "build": "tsc -p tsconfig.json",
    "clean": "git checkout HEAD -- sandbox && git clean -f -d sandbox",
    "launch": "cd sandbox && ng g schematic-collection:schematic-name",
    "clean:launch": "yarn clean && yarn launch",
    "build:clean:launch": "yarn build && yarn clean:launch",
    "test": "yarn build:clean:launch && yarn test:sandbox && yarn clean",
    "test:sandbox": "cd sandbox && yarn lint && yarn test && yarn e2e && yarn build",
    "link:schematic": "yarn link && cd sandbox && yarn link schematic-collection",
    "publish": "yarn test && PUBLISH_CMD"

Build the schematic

You may already have this script, but for this post we’ll assume that your script to build the schematic is as follows. This will compile the Typescript files to Javascript.

"build": "tsc -p tsconfig.json"

Clean up

Since schematics often make file system changes to your sandbox code, let’s start with resetting the sandbox before we begin executing the schematic. Add a clean script that resets the sandbox to the version-controlled state.

"clean": "git checkout HEAD -- sandbox && git clean -f -d sandbox"

Link the sandbox

This sandbox pattern only works if we can execute the schematic locally against another local application. The link:schematic script will create a symlink of the schematic to your global package directory under the package.json['name'] value. We will then cd into the sandbox and link to the schematic package name. Now, whenever that package is requested inside the sandbox it will execute the code one directory up in ./src.

"link:schematic": "yarn link && cd sandbox && yarn link schematic-collection"

You can verify this linking succeeded by checking the sandbox's node_modules under the schematic's package name.

Run the schematic

Next, let’s configure how we’ll test the schematic against the sandbox. First, we’ll create the script to run the schematic which we’ll reuse later. The script will cd into the sandbox and run the generate command from the Angular CLI. You can pass any options your schematic requires if necessary.

"launch": "cd sandbox && ng g schematic-collection:schematic-name"

If you’re taking advantage of Schematic prompts you can inline the options to avoid bring prompted and create a separate launch script to test the prompts.

Avoid CLI prompts with inline options

"launch": "cd sandbox && ng g schematic-collection:schematic-name --name=hello"

Now that we have our launch command lets create a script that resets the sandbox before running the schematic. We’ll combine two scripts that we’ve made so far, clean and launch. This is helpful when you need to rerun the schematic without changing the schematic code.

"clean:launch": "yarn clean && yarn launch"

Finally, an action you’ll likely perform the most will build the schematic, reset the sandbox, and run the schematic in the sandbox.

"build:clean:launch": "yarn build && yarn clean:launch"

You can also pass options to the wrapped launch command by appending them to the yarn/npm script:

yarn build:clean:launch --name=hello


So far we’ve configured how to build, and run our schematic as well as reset the sandbox back to its "initial" state. This provides us with the ability to visually see what our schematic is doing against the sandbox application. We can improve this further, though. For example, after our schematic runs, it might be a good idea to make sure the app still operates normally. One way to test this is to run ng {test, e2e, lint, build} in the sandbox application.

To do that let’s create a script that’s similar to what we did with launch.

"test:sandbox": "cd sandbox && yarn lint && yarn test && yarn e2e && yarn build"

Now let's wrap up some additional functionality into a single script. Here we’ll do a build, clean, and launch. Then we’ll run the tests we specified in test:sandbox and finally we’ll reset the sandbox after the test in case we’re running this locally with clean.

"test": "yarn build:clean:launch && yarn test:sandbox && yarn clean"

We now have a single script to test our Schematic end to end. This is useful if you have a release process that runs test(s) before publishing. Now you can rely on this script to pass/fail your release and you’re on your way to CI/CD bliss. 🙂


Eventually you’re going to want to publish your schematic for the world to use. Fortunately we’ve added the functionality to develop faster and with better confidence by being able to see what our schematic is doing. We’ve also orchestrated our scripts in a way that makes it trivial to confidently publish our schematic. Although everyone’s publishing needs vary, it’s now easy to configure a publishing script.

"publish": "yarn test && PUBLISH_CMD"

Additional Configuration

Schematic names

Schematic projects are capable of containing a collection of schematics. Which is why you can call your schematic like

ng g schematic-collection:schematic-name-a
ng g schematic-collection:schematic-name-b

In the schematic .src/SCHEMATIC/collection.json you can configure the public API of your schematic code. Specifying properties in the schematics object will make them available after the : from the example above.

    "$schema": "../node_modules/@angular-devkit/schematics/collection-schema.json",
    "schematics": {
        "prettier": {
            "description": "AddsPrettier to the application.",
            "factory": "./prettier/index",
            "schema": "./prettier/schema.json"
        "add": {
            "description": "AddsPrettier to the application.",
            "factory": "./prettier/index",
            "schema": "./prettier/schema.json"
        "ng-add": {
            "description": "AddsPrettier to the application.",
            "factory": "./prettier/index",
            "schema": "./prettier/schema.json"

The following schematic names would be available with the above configuration

ng g @schuchard/prettier:add
ng g @schuchard/prettier:prettier

A special property name is ng-add. Specifying this property and pointing the factory to a schematic will allow a CLI consumer to access the schematic in this following manner.

ng add schematic-collection

# or

ng add @schuchard/prettier

Note that currently, ng add will execute the schematic and add it as a dependency of the application it’s run against.

Helpful resources and links

Want to test out everything mentioned above? Fork this repository

Just getting started with schematics? This is a great article to read.

Looking for inspiration or guidance on writing schematics? The Angular CLI source code is an excellent resource.

Here are a few schematics I’ve written that use the Sandbox testing approach. Note that some of the scripts are named slightly different as this process has evolved over time.

Interested in CLI prompts? I have an article on that as well.