React Hooks | Why useCallback ?

The Problem

Functions are First-class citizens/objects in JS , which simply means they behave like objects , functions can be ~ stored in variable , passed as argument or returned from a function.

// Referential Equality  ( () => {} ) === ( () => {} ) // false  { greeting:"Hi"} === { greeting:"Hi"} //false 

Components Re-rendering

In React rendering means creating react elements and displaying on screen , this can be caused by state change, context change or a parent re-render. This is will cause re-run of the functional Component which will cause any function inside the Component to be destroyed (Garbage Collected) and created again on each re-render.

Example

Below Component renders a Text and button fields. value from the text field is saved to localStorage onChange.

import React from "react"; const fnCount = new Set(); // stores only unique values export default function App() { const [name, setName] = React.useState( () => window.localStorage.getItem("name") || "" ); const [count, setCount] = React.useState(0); const saveText = () => { window.localStorage.setItem("name", name); }; React.useEffect(() => { saveText(); }, [saveText]); fnCount.add(saveText); // keeps count of times new fn created console.log(fnCount); return ( <div className="App"> <h1>Understanding useCallback</h1>  <input type="text" value={name} onChange={(e) => setName(e.target.value)} placeholder="Your Name :" /> <button onClick={() => { setCount((c) => c + 1); }} > Count:{count} </button>  </div>  ); } 

In above code fnCount is a Set so it will keep track of number of times function saveText is created on each re-render of App component triggered on change of Input field.

Below is the console log after 3 button clicks

On Component mount, fnCount had one 1 entry but on each re-render of component saveText fn is re-initialized and is brand new so now fnCount has 4 entries (1 + 3 for each re-render). Since saveText is related to name state so should not be re-run on button click. This is the problem useCallback solves by not letting React to initialize function on each render unless dependencies in useCallback changes.

Solution

 // const saveText = () => { // window.localStorage.setItem("name", name); // }; const saveText = React.useCallback(() => { window.localStorage.setItem("name", name); }, [name]); 

After wrapping saveText fn with useCallback , function count for saveText remains 1 in fnCount (mount time) after 3 clicks.
Since on button Click no dependencies of saveText is changed so
useCallback returns the memoized (or memorized) function , that was returned last time.

Example App :