useCallback

Before addressing this question, I was studying the situation where functions enter the dependency array of useEffect.

Typically, when creating a custom hook, the hook receives state values from the component’s props and uses them in the dependency array of useEffect for handling operations

So, what happens when a function is received as a prop? I contemplated this.

Why Functions Are Included in the Dependency Array

Whenever a React component renders, functions are recreated.

In JavaScript, since functions are first-class objects, a function with the same name is treated as a different function each time it is recreated during rendering.

Therefore, if a function is included in the dependency array, useEffect will run again every time that function is recreated during rendering.

Example Using useCallback

import React, { useState, useEffect, useCallback } from 'react'

function SearchComponent() {
  const [searchQuery, setSearchQuery] = useState('')
  const [results, setResults] = useState([])

  const fetchData = useCallback(() => {
    fetch(`https://api.example.com/search?q=${searchQuery}`)
      .then((response) => response.json())
      .then((data) => setResults(data.results))
      .catch((error) => console.error('Error fetching data:', error))
  }, [searchQuery]) // fetchData function depends on searchQuery.

  useEffect(() => {
    fetchData()
  }, [fetchData]) // fetchData is included in the dependency array.

  return (
    <div>
      <input
        type="text"
        value={searchQuery}
        onChange={(e) => setSearchQuery(e.target.value)}
      />
      <ul>
        {results.map((result, index) => (
          <li key={index}>{result.name}</li>
        ))}
      </ul>
    </div>
  )
}

export default SearchComponent

Example Without Using useCallback

import React, { useState, useEffect } from 'react'

function SearchComponent() {
  const [searchQuery, setSearchQuery] = useState('')
  const [results, setResults] = useState([])

  useEffect(() => {
    const fetchData = () => {
      fetch(`https://api.example.com/search?q=${searchQuery}`)
        .then((response) => response.json())
        .then((data) => setResults(data.results))
        .catch((error) => console.error('Error fetching data:', error))
    }

    fetchData()
  }, [searchQuery]) // useEffect runs every time searchQuery changes.

  return (
    <div>
      <input
        type="text"
        value={searchQuery}
        onChange={(e) => setSearchQuery(e.target.value)}
      />
      <ul>
        {results.map((result, index) => (
          <li key={index}>{result.name}</li>
        ))}
      </ul>
    </div>
  )
}

export default SearchComponent

Differences

1. Function Creation and Memory Usage

• When a function is defined directly inside useEffect, it is newly defined every time useEffect runs due to changes in searchQuery.
• Because the function is recreated each time, memory usage may increase, and constantly redefining the function can lead to unnecessary computations.
• This difference can impact performance-sensitive applications, particularly in components that render frequently.

2. Dependency Management

• If the function is not wrapped in useCallback, it is not included as a dependency in the dependency array.
• useEffect will only run when searchQuery changes. In this case, even though fetchData is newly defined every time searchQuery changes, dependency management is not complicated; it simply runs with the new function definition.

3. Preventing Unnecessary Re-renders

• When using useCallback, the function is only recreated when values in the dependency array change. This helps prevent unnecessary re-renders of the component.
• Without useCallback, the fetchData function is recreated every time, leading to potential unintended re-renders in parts of the component managing other dependencies.

When to Use useCallback

1. Complex Components

• When managing many states: The component becomes complex when managing multiple states that interact with each other. For example, this occurs in form components with many input fields or UIs that dynamically change based on various events.
• When there are many child components: The complexity increases when a component renders multiple child components that exchange a lot of data, especially if those child components manage independent states or share the parent component’s state.
• When there is a lot of conditional rendering: The code can become complex with multiple conditional statements (if, switch, ternary operators) when rendering different UIs based on various conditions.

2. Performance-Sensitive Situations

• Frequently rendered components: These components need to respond quickly, such as real-time updating dashboards, chat applications, or UIs with many animations.
• Handling large amounts of data: This situation arises when a component needs to process or render a lot of data at once, like rendering thousands of rows in a table or creating complex graphs.
• When optimization is needed: If certain operations occur frequently and negatively impact performance, optimization is necessary. This includes using React’s optimization hooks like React.memo, useCallback, and useMemo to reduce unnecessary re-renders.

In these situations, if the component doesn’t operate efficiently, user experience can suffer, making it essential to optimize code or leverage React’s performance-related features.

Conclusion

• When Not Using useCallback: Defining and executing a function directly inside useEffect can lead to the function being recreated every time a value in the dependency array changes, impacting memory and performance. However, in simple situations, this might not be a significant issue.
• When Using useCallback: It helps prevent unnecessary re-creations of functions, contributing to performance optimization. This method is especially efficient if functions are frequently recreated.

Thus, while there may not be a significant difference in small-scale components, using useCallback to memoize functions can be a better choice in complex components or performance-sensitive scenarios.