Great question! Handling multiple states with useEffect is a common point of confusion, so let's break down the core best practices and tradeoffs between the two approaches you've outlined.

Core Best Practices to Follow First #

Before diving into the two approaches, here are some foundational rules to keep in mind:

• Keep effects focused: Each effect should handle one logical concern. This makes your code easier to debug, test, and maintain as your app grows.
• Always define a dependency array: Skipping this means your effect runs on every single render—a surefire way to introduce performance issues and unexpected behavior.
• Avoid mixing unrelated logic: If two state changes trigger completely separate actions (e.g., one fetches data, the other updates a UI tooltip), don't cram them into the same effect.

Deep Dive into the Two Approaches #

Approach 1: Single useEffect with Conditional Checks #

This approach lumps all state-dependent logic into one effect, using conditionals to run code only when specific states change.

Code Example:

// Note: Missing dependency array here—this runs on EVERY render!
useEffect(() => {
  if (first) {
    // Logic for when `first` state changes
  }
  if (second) {
    // Logic for when `second` state changes
  }
})

Pros

• Consolidated related logic: If the two state changes are part of a single workflow (e.g., updating a live form preview when either of two inputs changes), keeping them together makes the connection between states and actions clear.
• Shared setup/cleanup: If both logic paths rely on the same initialization or cleanup code (e.g., setting up a subscription that both states use), you avoid duplicating that code.

Cons

• Uncontrolled re-runs: Without a dependency array, this effect runs every time the component renders—even when neither first nor second changes. This wastes resources and can cause jank in UIs.
• Wasted conditional checks: Even if you add [first, second] to the dependency array, the effect runs whenever either state changes. That means if only first updates, the code still checks the second conditional (harmless, but unnecessary).
• Harder debugging: If there's a bug in one of the conditional blocks, you have to sift through more code to find it. React DevTools also won't clearly show which state triggered the effect.
• Messy cleanup: If each logical path needs its own cleanup, mixing them in one effect leads to tangled hard-to-maintain cleanup logic.

Approach 2: Separate Effects for Each State #

This approach splits logic into individual effects, each tied to its own state dependency.

Code Example:

useEffect(() => {
  // Logic for when `first` state changes
}, [first])

useEffect(() => {
  // Logic for when `second` state changes
}, [second])

Pros

• Clear responsibility: Each effect has one job. Any developer reading your code can immediately see which state triggers which action—no hunting through conditionals.
• Optimized execution: Each effect only runs when its specific dependency changes. No extra renders, no unnecessary conditional checks.
• Easier debugging: If something breaks with the first state's logic, you only need to look at the first effect. React DevTools will also explicitly show which effect triggered a re-run.
• Isolated cleanup: If an effect needs cleanup (e.g., canceling a fetch), you define it right in that effect, keeping cleanup tightly coupled to the code it supports.

Cons

• Potential code duplication: If the two effects share a lot of common code (e.g., calling the same utility function with different args), you might repeat yourself. The fix here is simple: extract the shared logic into a reusable helper function.
• Over-separation for coupled logic: If the two state changes are tightly linked (e.g., calculating a single derived value based on both states), splitting them into separate effects might make the relationship between states and the derived value less obvious.

When to Choose Which Approach? #

• Default to Approach 2: Separate effects align with React's official recommendations and make your code more maintainable long-term. Trust me, debugging a single overloaded effect is way more of a headache than splitting it up.
• Use Approach 1 sparingly: Only opt for a single effect with conditionals if the two state changes are part of a single cohesive workflow, and always add the proper dependency array ([first, second]) to avoid unnecessary re-runs. Even then, consider extracting shared logic into helpers to keep things clean.

内容的提问来源于stack exchange，提问作者Syed Aqeel