Pinia

Pinia is a tool for managing client-side state for Vue applications. Refer to the official documentation on how to use Pinia.

Best practices

Pinia instance

You should always prefer using shared Pinia instance from ~/pinia/instance. This allows you to easily add more stores to components without worrying about multiple Pinia instances.

import { pinia } from '~/pinia/instance';

new Vue({ pinia, render(h) { return h(MyComponent); } });

Small stores

Prefer creating small stores that focus on a single task only. This is contrary to the Vuex approach which encourages you to create bigger stores.

Treat Pinia stores like cohesive components rather than giant state façades (Vuex modules).

Vuex design ❌

flowchart TD
    A[Store]
    A --> B[State]
    A --> C[Actions]
    A --> D[Mutations]
    A --> E[Getters]
    B --> F[items]
    B --> G[isLoadingItems]
    B --> H[itemWithActiveForm]
    B --> I[isSubmittingForm]

Pinia's design ✅

flowchart TD
    A[Items Store]
    A --> B[State]
    A --> C[Actions]
    A --> D[Getters]
    B --> E[items]
    B --> F[isLoading]

    H[Form Store]
    H --> I[State]
    H --> J[Actions]
    H --> K[Getters]
    I --> L[activeItem]
    I --> M[isSubmitting]

Single file stores

Place state, actions, and getters in a single file. Do not create 'barrel' store index files which import everything from actions.js, state.js and getters.js.

If your store file gets too big it's time to consider splitting that store into multiple stores.

Use Option Stores

Pinia offers two types of store definitions: option and setup. Prefer the option type when creating new stores. This promotes consistency and will simplify the migration path from Vuex.

Global stores

Prefer using global Pinia stores for global reactive state.

// bad ❌
import { isNarrowScreenMediaQuery } from '~/lib/utils/css_utils';

new Vue({
  data() {
    return {
      isNarrow: false,
    };
  },
  mounted() {
    const query = isNarrowScreenMediaQuery();
    this.isNarrow = query.matches;

    query.addEventListener('change', (event) => {
      this.isNarrow = event.matches;
    });
  },
  render() {
    if (this.isNarrow) return null;
    //
  },
});

// good ✅
import { pinia } from '~/pinia/instance';
import { useViewport } from '~/pinia/global_stores/viewport';

new Vue({
  pinia,
  ...mapState(useViewport, ['isNarrowScreen']),
  render() {
    if (this.isNarrowScreen) return null;
    //
  },
});

Hot Module Replacement

Pinia offers an HMR option that you have to manually attach in your code. The experience Pinia offers with this method is subpar and this should be avoided.

Testing Pinia

Unit testing the store

Follow the official testing documentation.

Official documentation suggests using setActivePinia(createPinia()) to test the Pinia.

Our recommendation is to leverage createTestingPinia with unstubbed actions. It acts the same as setActivePinia(createPinia()) but also allows us to spy on any action by default.

Always use createTestingPinia with stubActions: false when unit testing the store.

A basic test could look like this:

import { createTestingPinia } from '@pinia/testing';
import { useMyStore } from '~/my_store.js';

describe('MyStore', () => {
  beforeEach(() => {
    createTestingPinia({ stubActions: false });
  });

  it('does something', () => {
    useMyStore().someAction();
    expect(useMyStore().someState).toBe(true);
  });
});

Any given test should only check for any of these three things:

1. A change in the store state
2. A call to another action
3. A call to side effect (for example an Axios request)

Never try to use the same Pinia instance in more than one test case. Always create a fresh Pinia instance because it is what actually holds your state.

Unit testing components with the store

Follow the official testing documentation.

Pinia requires special handling to support Vue 3 compat mode:

1. It must register PiniaVuePlugin on the Vue instance
2. Pinia instance must be explicitly provided to the shallowMount/mount from Vue Test Utils
3. Stores have to be created prior to rendering the component, otherwise Vue will try to use Pinia for Vue 3

A full setup looks like this:

import Vue from 'vue';
import { createTestingPinia } from '@pinia/testing';
import { PiniaVuePlugin } from 'pinia';
import { shallowMount } from '@vue/test-utils';
import { useMyStore } from '~/my_store.js';
import MyComponent from '~/my_component.vue';

Vue.use(PiniaVuePlugin);

describe('MyComponent', () => {
  let pinia;
  let wrapper;

  const createComponent = () => {
    wrapper = shallowMount(MyComponent, { pinia });
  }

  beforeEach(() => {
    pinia = createTestingPinia();
    // store is created before component is rendered
    useMyStore();
  });

  it('does something', () => {
    createComponent();
    // all actions are stubbed by default
    expect(useMyStore().someAction).toHaveBeenCalledWith({ arg: 'foo' });
    expect(useMyStore().someAction).toHaveBeenCalledTimes(1);
  });
});

In most cases you won't need to set stubActions: false when testing components. Instead, the store itself should be properly tested and the component tests should check that the actions were called with correct arguments.

Setting up initial state

Pinia doesn't allow to unstub actions once they've been stubbed. That means you can not use them to set the initial state if you didn't set stubActions: false.

In that case it is allowed to set the state directly:

describe('MyComponent', () => {
  let pinia;
  let wrapper;

  const createComponent = () => {
    wrapper = shallowMount(MyComponent, { pinia });
  }

  beforeEach(() => {
    // all the actions are stubbed, we can't use them to change the state anymore
    pinia = createTestingPinia();
    // store is created before component is rendered
    useMyStore();
  });

  it('does something', () => {
    // state is set directly instead of using an action
    useMyStore().someState = { value: 1 };
    createComponent();
    // ...
  });
});

Migrating from Vuex

GitLab is actively migrating from Vuex, you can contribute and follow this progress in epic 18476.

Before migrating decide what your primary state manager should be first. Proceed with this guide if Pinia was your choice.

Migration to Pinia could be completed in two ways: a single step migration and a multi-step one.

Follow single step migration if your store meets these criteria:

1. Store contains only one module
2. Actions, getters and mutations cumulatively do not exceed 1000 lines

In any other case prefer the multi-step migration.

Single step migration

Follow the official Vuex migration guide.

1. Migrate store to Pinia using codemods
2. Fix store tests following our guide and best practices
3. Update components to use the migrated Pinia store
   1. Replace mapActions, mapState with Pinia counterparts
   2. Replace mapMutations with Pinia's mapActions
   3. Replace mapGetters with Pinia's mapState
4. Fix components tests following our guide and best practices

If your diff starts to exceed reviewable size prefer the multi-step migration.

Multi-step migration

Learn about the official Vuex migration guide.

A walkthrough is available in the two part video series:

1. Migrating the store (part 1)
2. Migrating the components (part 2)

Follow these steps to iterate over the migration process and split the work onto smaller merge requests:

1. Identify the store you are going to migrate. Start with the file that defines your store via new Vuex.Store() and go from there. Include all the modules that are used inside this store.

2. Create a migration issue, assign a migration DRI(s) and list all the store modules you're going to migrate. Track your migration progress in that issue. If necessary, split the migration into multiple issues.

3. Create a new CODEOWNERS (.gitlab/CODEOWNERS) rule for the store files you're migrating, include all the Vuex module dependencies and store specs.

   If you are migrating only a single store module then you would need to include only state.js (or your index.js), actions.js, mutations.js and getters.js and their respective spec files.

   Assign at least two individuals responsible for reviewing changes made to the Vuex store. Always sync your changes from Vuex store to Pinia. This is very important so you don't introduce regressions with the Pinia store.

4. Copy existing store as-is to a new location (you can call it stores/legacy_store for example). Preserve the file structure. Do this for every store module you're going to migrate. Split this into multiple merge requests if necessary.

5. Create an index file (index.js) with a store definition (defineStore) and define your state in there. Copy the state definition from state.js. Do not import actions, mutations and getters yet.

6. Use code mods to migrate the store files. Import migrated modules in your new store's definition (index.js).

7. If you have circular dependencies in your stores consider using tryStore plugin.

8. Migrate the store specs manually.

9. Sync your Vuex store with Pinia stores.

10. Refactor components to use the new store. Split this into as many merge requests as necessary. Always update the specs with the components.

11. Remove the Vuex store.

12. Remove CODEOWNERS rule.

13. Close the migration issue.

Example migration breakdown

You can use the merge requests migration breakdown as a reference:

1. Diffs store
   1. Copy store to a new location and introduce CODEOWNERS rules
   2. Automated store migration
      1. Also creates MrNotes store
   3. Specs migration (actions, getters, mutations)
2. Notes store
   1. Copy store to a new location
   2. Automated store migration
   3. Specs migration (actions, getters, mutations)
3. Batch comments store
   1. Copy store to a new location
   2. Automated store migration
   3. Specs migration (actions, getters, mutations)
4. Sync Vuex stores with Pinia stores
5. Diffs store components migration
   1. Diffs app
   2. Non diffs components
   3. File browser
   4. Diffs components
   5. Diff file components
   6. Rest of diffs components
6. Batch comments components migration
7. MrNotes components migration
8. Notes store components migration
   1. Diffs components
   2. Simple notes components
   3. More notes components
   4. Rest of notes components
   5. Notes app
9. Remove Vuex from merge requests
   1. Also removes the CODEOWNERS rules

Post migration steps

Once your store is migrated consider refactoring it to follow our best practices. Split big stores into smaller ones. Refactor tryStore uses.

Automated migration using codemods

You can use ast-grep codemods to simplify migration from Vuex to Pinia.

1. Install ast-grep on your system before proceeding.
2. Run scripts/frontend/codemods/vuex-to-pinia/migrate.sh path/to/your/store

The codemods will migrate actions.js, mutations.js and getters.js located in your store folder. Manually scan these files after running the codemods to ensure they are properly migrated. Vuex specs can not be automatically migrated, migrate them by hand.

Vuex module calls are replaced using Pinia conventions:

If you have not yet migrated a dependent module (useAnotherModule and useRootStore in the examples above) you can create a temporary dummy store. Use the guidance below to migrate Vuex modules.

Migrating stores with nested modules

It is not trivial to iteratively migrate stores with nested modules that have dependencies between them. In such cases prefer migrating nested modules first:

1. Create a Pinia store counterpart of the nested Vuex store module.
2. Create a placeholder Pinia 'root' store for root module dependencies if applicable.
3. Copy and adapt existing tests for the migrated module.
4. Do not use migrated modules yet.
5. Once all the nested modules are migrated you can migrate the root module and replace the placeholder store with the real one.
6. Replace Vuex store with Pinia stores in components.

Avoiding circular dependencies

It is imperative that you don't create circular dependencies in your Pinia stores. Unfortunately Vuex design allows to create interdependent modules that we have to refactor later.

An example circular dependency in store design:

graph TD
    A[Store Alpha] --> Foo(Action Foo)
    B[Store Beta] --> Bar(Action Bar)
    A -- calls --> Bar
    B -- calls --> Foo

To mitigate this issue consider using tryStore plugin for Pinia during migration from Vuex:

Before

// store_alpha/actions.js
function callOtherStore() {
  // bad ❌, circular dependency created
  useBetaStore().bar();
}

// store_beta/actions.js
function callOtherStore() {
  // bad ❌, circular dependency created
  useAlphaStore().bar();
}

After

// store_alpha/actions.js
function callOtherStore() {
  // OK ✅, circular dependency avoided
  this.tryStore('betaStore').bar();
}

// store_beta/actions.js
function callOtherStore() {
  // OK ✅, circular dependency avoided
  this.tryStore('alphaStore').bar();
}

This will look up the store by its name using Pinia instance and prevent the circular dependency issue. Store name is defined when calling defineStore('storeName', ...).

You must initialize both stores prior to component mounting when using tryStore:

// stores are created in advance
useAlphaStore();
useBetaStore();
new Vue({ pinia, render(h) { return h(MyComponent); } });

The tryStore helper function can only be used during migration. Never use this in proper Pinia stores.

Refactoring tryStore

After you finished the migration it is very important to redesign the stores so there are no more circular dependencies.

The easiest way to solve this would be to create a top level store that would orchestrate other stores.

Before

graph TD
    A[Store Alpha] --> Foo(Action Foo)
    A -- calls --> Bar
    B[Store Beta] --> Bar(Action Bar)
    B -- calls --> Foo

After

graph TD
    C[Store Gamma]
    A[Store Alpha] --- Bar(Action Bar)
    B[Store Beta] --- Foo(Action Foo)
    C -- calls --> Bar
    C -- calls --> Foo

Syncing with Vuex

This syncWithVuex plugin syncs your state from Vuex to Pinia and vice versa. This allows you to iteratively migrate components by having both stores in your app during migration.

Usage example:

// Vuex store @ ./store.js
import Vuex from 'vuex';
import createOldStore from './stores/old_store';

export default new Vuex.Store({
  modules: {
    oldStore: createOldStore(),
  },
});

// Pinia store
import { defineStore } from 'pinia';
import oldVuexStore from './store'

export const useMigratedStore = defineStore('migratedStore', {
  syncWith: {
    store: oldVuexStore,
    name: 'oldStore', // use legacy store name if it is defined inside Vuex `modules`
    namespaced: true, // set to 'true' if Vuex module is namespaced
  },
  // the state here gets sync with Vuex, any changes to migratedStore also propagate to the Vuex store
  state() {
    // ...
  },
  // ...
});

Override

A Vuex store definition can be shared in multiple Vuex store instances. In that case we can not rely on the store config alone to sync our Pinia store with the Vuex store. We need to point our Pinia store to the actual Vuex store instance using syncWith helper function.

// this overrides the existing `syncWith` config
useMigratedStore().syncWith({ store: anotherOldStore });
// `useMigratedStore` state now is synced only with `anotherOldStore`
new Vue({ pinia, render(h) { return h(MyComponent) } });

Migrating store tests

testAction

Some Vuex tests might use testAction helper to test that certain actions or mutations have been called. We can migrate these specs using createTestPiniaAction helper from helpers/pinia_helpers in Jest.

Before

describe('SomeStore', () => {
  it('runs actions', () => {
    return testAction(
      store.actionToBeCalled, // action to be called immediately
      { someArg: 1 }, // action call arguments
      { someState: 1 }, // initial store state
      [{ type: 'MUTATION_NAME', payload: '123' }], // mutation calls to expect
      [{ type: 'actionName' }], // action calls to expect
    );
  });
});

After

import { createTestPiniaAction } from 'helpers/pinia_helpers';

describe('SomeStore', () => {
  let store;
  let testAction;

  beforeEach(() => {
    store = useMyStore();
    testAction = createTestPiniaAction(store);
  });

  it('runs actions', () => {
    return testAction(
      store.actionToBeCalled,
      { someArg: 1 },
      { someState: 1 },
      [{ type: store.MUTATION_NAME, payload: '123' }], // explicit reference to migrated mutation
      [{ type: store.actionName }], // explicit reference to migrated action
    );
  });
});

Avoid using testAction in your proper Pinia tests: this should only be used during migration. Always prefer testing each action call explicitly.

Custom getters

Pinia allows to define custom getters in Vue 3. Since we're using Vue 2 this is not possible. To work around this you can use createCustomGetters helper from helpers/pinia_helpers.

Before

describe('SomeStore', () => {
  it('runs actions', () => {
    const dispatch = jest.fn();
    const getters = { someGetter: 1 };
    someAction({ dispatch, getters });
    expect(dispatch).toHaveBeenCalledWith('anotherAction', 1);
  });
});

After

import { createCustomGetters } from 'helpers/pinia_helpers';

describe('SomeStore', () => {
  let store;
  let getters;

  beforeEach(() => {
    getters = {};
    createTestingPinia({
      stubActions: false,
      plugins: [
        createCustomGetters(() => ({
          myStore: getters, // each store used in tests should be also declared here
        })),
      ],
    });
    store = useMyStore();
  });

  it('runs actions', () => {
    getters.someGetter = 1;
    store.someAction();
    expect(store.anotherAction).toHaveBeenCalledWith(1);
  });
});

Avoid mocking getters in proper Pinia tests: this should only be used for migration. Instead, provide a valid state so a getter can return correct value.

Migrating component tests

Pinia does not return promises in actions by default. Because of that pay a special attention when using createTestingPinia. Since it stubs all the actions it does not guarantee that an action would return a promise. If your component's code is expecting an action to return a promise stub it accordingly.

describe('MyComponent', () => {
  let pinia;

  beforeEach(() => {
    pinia = createTestingPinia();
    useMyStore().someAsyncAction.mockResolvedValue(); // this now returns a promise
  });
});