useEffect's Dependency Array

The dependency array of useEffect defines the variables or values that the effect depends on. When any of these dependencies change, the effect will be re-run. Here are some examples of what can be included in the dependency array:

1. State variables

   Any state variable in your component can be included as a dependency. This allows the effect to re-run when the state changes.

Example:

  useEffect(() => {
    // Effect runs when `searchQuery` changes
  }, [searchQuery]);

2. Props

If your component receives props, you can include them in the dependency array. This ensures that the effect re-runs when props change.

Example:

useEffect(() => {
  // Effect runs when `userId` prop changes
}, [userId]);

3. Computed values (derived from state or props)

   Any values derived from state or props, such as a calculated value or result of a function, can be dependencies.

   Example:

const derivedValue = state.someValue + props.someProp;
useEffect(() => {
  // Effect runs when derivedValue changes
}, [derivedValue]);

4. Functions

   Functions that are defined outside of the useEffect or inside the component (depending on whether the function's identity changes) can be dependencies.

   Example:

const handleClick = () => {
  // Some action
};
useEffect(() => {
  // Effect runs when `handleClick` function changes
}, [handleClick]);

5. Refs

   Refs can be included in the dependency array, but they generally don't trigger a re-render, so you typically only need to use refs in a way that doesn't affect the behavior of the effect directly. Use refs in dependency array sparingly.

   Example:

const inputRef = useRef();
useEffect(() => {
  // Effect runs when `inputRef` changes
}, [inputRef]);

6. Custom hooks

   If you use custom hooks that return state or values, those values may need to be included in the dependency array to ensure proper reactivity.

   Example:

const { data } = useCustomHook();
useEffect(() => {
  // Effect runs when data changes
}, [data]);

7. Array or Object Values

   If you are using arrays or objects as dependencies, ensure that they are not re-created on every render (such as when using useEffect or state updates that recreate them). Example:

const values = [1, 2, 3];
useEffect(() => {
  // Effect runs when `values` changes
}, [values]);

This may cause unnecessary re-renders.