OnPush in all components

If the user enters your site, and the site is sluggish and slow, the user will probably leave. We need to strive to make our web application as fast as possible. One way to speed you angular application is by using the OnPush change detection strategy on all components.
This best practice tip will force you to make all your components OnPush.

Generate a new angular application

npx @angular/cli@latest new all-onpush --minimal --style css --routing false

Angular cli is set to OnPush

Problem: if you generate a component with Angular cli, it will not be set to OnPush.
Make sure every new component that is generated with the angular cli is set to OnPush.

angular.json

...
"@schematics/angular:component": {
  ...
  "changeDetection": "OnPush"
},
...

Add Lint

Problem: if you remove the OnPush manually, or create a component without Angular cli, you might forget to set the OnPush change detection strategy. Lint can help you get a lint error on every component that is not set to OnPush.
Currently the app.component.ts is not set to OnPush, so let’s install eslint to help us catch those problems.

npx ng lint

will install lint and running the same command again will show you the lint error. currently we have no lint errors, let’s make lint error when we are not using OnPush.

Lint rule: @angular-eslint/prefer-on-push-component-change-detection

We need to tell ESLint to watch for components that do not use OnPush and give us a lint error.

".eslintrc.json

"rules": {
  "@angular-eslint/prefer-on-push-component-change-detection": [
    "error"
  ],
  ...
}

If you try and run lint again:

npx ng lint

you will see an error on the app.component.ts not using OnPush

Make sure your ci is running lint

Configure your CI to run lint, so you will not be able to merge code that is not using OnPush.
In this example on every PR github actions will run ng lint.

.github/workflows/lint.yml

name: Lint

on:
  push:
    branches: [main]
  pull_request:
    branches: [main]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Lint
        uses: actions/setup-node@v3
        with:
          node-version: 18
      - run: npm ci
      - run: npx ng lint

There is a significant performance boost when using OnPush change detection strategy, let’s change all our components to use OnPush, and also enforce it with lint.