createSelector

function createSelector<T, U>(
  source: () => T,
  fn?: (a: U, b: T) => boolean
): (k: U) => boolean

Creates a conditional signal that only notifies subscribers when entering or exiting their key matching the value. Useful for delegated selection state. As it makes the operation O(1) instead of O(n).

const isSelected = createSelector(selectedId)

<For each={list()}>
  {(item) => <li classList={{ active: isSelected(item.id) }}>{item.name}</li>}
</For>

In the above code if the item.id is equal to the selectedId the active class will be added to the li element. If the item.id is not equal to the selectedId the active class will be removed from the li element.

Arguments