Angular 20 Vitest Setup

Use this workflow for existing Angular 20 apps that need Vitest as the unit test runner.

Workflow

1. Run deterministic preflight checks from the Angular workspace root.

• Confirm angular.json exists.
• Confirm package.json exists.
• Confirm the target project is identified:
  • If the user names a project, use that project.
  • Otherwise, if exactly one project exists, use it.
  • Otherwise, stop and ask which project to migrate.
• Confirm the target project has a test target under either architect.test or targets.test.
• If any check fails, stop and report the exact missing prerequisite.

2. Validate Angular compatibility before changing config.

• Ensure the workspace uses Angular tooling compatible with @angular/build:unit-test (Angular 20 workflow target).
• If incompatible, stop and report the detected version/tooling mismatch.

3. Install Vitest dependencies.

npm install --save-dev vitest jsdom

• If dependencies are already present, do not reinstall unnecessarily.

4. Configure the project test target to use the Angular unit-test builder.

• Set projects.<project-name>.architect.test.builder to @angular/build:unit-test when architect is used.
• Set projects.<project-name>.targets.test.builder to @angular/build:unit-test when targets is used.
• Keep test.options.tsConfig as-is when valid; otherwise set it to tsconfig.spec.json.
• Remove Karma-specific test options when present (for example karmaConfig, browser launcher options, or Karma-only polyfills such as zone.js/testing).
• Preserve unrelated test options.

Reference configuration:

"test": {
  "builder": "@angular/build:unit-test",
  "options": {
    "tsConfig": "tsconfig.spec.json"
  }
}

5. Remove Karma/Jasmine dependencies safely.

npm uninstall karma karma-chrome-launcher karma-coverage karma-jasmine karma-jasmine-html-reporter jasmine-core @types/jasmine

• In multi-project workspaces, remove shared dependencies only when no remaining project still requires Karma/Jasmine.
• If shared usage is uncertain, report the constraint and skip destructive removal.

6. Update spec files only when they still use Jasmine-only APIs.

• Replace Jasmine globals/matchers with Vitest-compatible APIs.
• Keep test behavior unchanged while refactoring assertions/spies.
• If no Jasmine-only usage remains, skip spec edits.

Verification

Run from workspace root:

npm run build
npm run test -- --watch=false

When possible, prefer scoped verification for the target project (for example Angular CLI project flags) to avoid unrelated failures.

Then verify all of the following:

• Target project test builder is @angular/build:unit-test in angular.json.
• vitest and jsdom exist in devDependencies.
• Karma/Jasmine packages are removed or explicitly documented as intentionally retained for other projects.
• Tests execute without Jasmine global references like jasmine..

Failure Handling

• If preflight fails, stop before installs/config edits and report exact blockers.
• If build/test fails after migration, report failing command, key error lines, and whether rollback was avoided.
• Do not modify unrelated projects unless explicitly requested.

Assistant Portability Rules

• Resolve and mutate only the targeted Angular project path in angular.json.
• Keep dependency cleanup conservative in multi-project workspaces; skip destructive removals when shared usage is unclear.
• Prefer deterministic command output in reports: exact command, exact path changed, exact blocker when stopped.

Output Requirements

When completing this skill, report:

1. Target project and exact angular.json test-target path changed.
2. Dependencies added, removed, and intentionally retained (if any).
3. Verification commands run and their outcomes.
4. Remaining migration blockers and next required action.

References