Custom isEqual camparison

By default track uses Object.is to compare values. But this might not be useful in all cases. Nalanda allows you to pass a custom isEqual function to a field to decide when a slice field has changed and the effect should be re-run.

const userNameField = key.field('userName', {
  equal: (a, b) => a.toLocaleLowerCase() === b.toLocaleLowerCase(),
})

Not tracking changes

While re-running effect whenever a slice field changes is helpful, sometimes you just want to read the state without tracking it. You can do this by using the get method instead of track.

effect((store) => {
  // effect will rerun only when 'isLoggedIn' changes
  const { isLoggedIn } = loginSlice.track(store);
  // effect will not rerun when 'userName' changes
  const { userName } = userSlice.get(store.state);
 
  if (isLoggedIn) {
    console.log(`The user ${userName} is logged in`);
  } else {
    console.log(`Please login ${userName}`);
  }
});

Running Effects only once

To do this, you can avoid using track and instead use get or getField to read the state. This will not track any changes and the effect will run only once.

Conditional tracking

If a condition is false, any tracking inside the condition will not be tracked (effects re-run will not depend on it) until the condition becomes true.

In the following case when isLoggedIn becomes true, the effect will start tracking userName i.e. it will re-run whenever userName changes.

effect((store) => {
  const { isLoggedIn } = loginSlice.track(store);
 
  if (isLoggedIn) {
    const { userName } = userSlice.track(store);
  }
});