# Effect

Setup logic that can hydrate an atom or react to atom changes.

Source: docs/source/concepts/effect.md
Packages: atom.io
Related: atom, catch, store

An atom effect is setup logic attached to an atom declaration.

Effects can hydrate an atom with `setSelf`, observe changes with `onSet`, or
connect atom state to an external system such as storage, URLs, or a service
boundary.

An effect may return a cleanup function. atom.io calls that cleanup when the atom
instance is disposed, which is where browser listeners, intervals, subscriptions,
or external handles should be released.

Use effects when state needs lifecycle-aware integration behavior that belongs
with the atom declaration.