Why does an effect run twice in development or loop forever?

Definition:

React Strict Mode performs an extra development-only setup and cleanup cycle for effects to reveal missing cleanup. An infinite effect loop is a different issue, usually caused by updating state that keeps changing an effect dependency. Understanding the trigger helps fix the design rather than masking the behavior.

Key points:

• Strict Mode runs an extra setup-and-cleanup cycle in development to expose bugs.
• A loop often means the effect changes state that changes its own dependencies.
• Fix the dependency or synchronization design; do not simply remove dependencies.

Reference: React useEffect.

• An object or function created during every render has a new identity and can retrigger an effect that depends on it.

Interview point:

• Do not claim production always runs every effect twice, and do not remove required dependencies just to silence a loop.