| 1 | --- |
| 2 | title: Initialize App Once, Not Per Mount |
| 3 | impact: LOW-MEDIUM |
| 4 | impactDescription: avoids duplicate init in development |
| 5 | tags: initialization, useEffect, app-startup, side-effects |
| 6 | --- |
| 7 | |
| 8 | ## Initialize App Once, Not Per Mount |
| 9 | |
| 10 | Do not put app-wide initialization that must run once per app load inside `useEffect([])` of a component. Components can remount and effects will re-run. Use a module-level guard or top-level init in the entry module instead. |
| 11 | |
| 12 | **Incorrect (runs twice in dev, re-runs on remount):** |
| 13 | |
| 14 | ```tsx |
| 15 | function Comp() { |
| 16 | useEffect(() => { |
| 17 | loadFromStorage() |
| 18 | checkAuthToken() |
| 19 | }, []) |
| 20 | |
| 21 | // ... |
| 22 | } |
| 23 | ``` |
| 24 | |
| 25 | **Correct (once per app load):** |
| 26 | |
| 27 | ```tsx |
| 28 | let didInit = false |
| 29 | |
| 30 | function Comp() { |
| 31 | useEffect(() => { |
| 32 | if (didInit) return |
| 33 | didInit = true |
| 34 | loadFromStorage() |
| 35 | checkAuthToken() |
| 36 | }, []) |
| 37 | |
| 38 | // ... |
| 39 | } |
| 40 | ``` |
| 41 | |
| 42 | Reference: [Initializing the application](https://react.dev/learn/you-might-not-need-an-effect#initializing-the-application) |
| 43 |