Equality and update control

By default, Veles uses referential equality (===) to decide whether something changed.

That applies to:

• set
• update
• derived state updates
• selector-based subscriptions
• selector-based rendering

Custom equality in derived state

Derived methods accept an equality option.

state.map

const parity$ = number$.map(
  (value) => ({ parity: value % 2 }),
  {
    equality: (a, b) => a.parity === b.parity,
  },
);

state.filter

const activeTask$ = task$.filter(
  (task) => task.active,
  {
    equality: (a, b) => a.id === b.id,
  },
);

state.scan

const summary$ = number$.scan(
  (acc, value) => ({ parity: (acc.parity + value) % 2 }),
  { parity: 0 },
  {
    equality: (a, b) => a.parity === b.parity,
  },
);

Comparator in subscriptions

track and trackSelected accept comparator options.

state.trackSelected(
  (value) => value.user,
  (user) => {
    console.log(user);
  },
  {
    comparator: (a, b) => a.id === b.id,
  },
);

Comparator in rendering

render and renderSelected also accept comparators.

state.renderSelected(
  (value) => value.user,
  (user) => <UserCard user={user} />,
  (a, b) => a.id === b.id,
);

When to use custom equality

Custom equality is useful when:

• you create new objects often
• only some fields matter for rendering
• you want to avoid unnecessary derived updates
• you want to avoid unnecessary DOM replacements

If simple referential equality is enough, prefer the default behavior.

Disabling equality

To disable equality completely, simply pass () => false function for equality checks, and it will always call all the subscribers.