ReactJS | Keys

11 May, 2022
In the previous article on ReactJS | Lists, we had discussed Keys and also told why they are needed while creating lists. We will continue the discussion further in this article.
A “key” is a special string attribute you need to include when creating lists of elements in React. Keys are used to React to identify which items in the list are changed, updated, or deleted. In other words, we can say that keys are used to give an identity to the elements in the lists. The next thing that comes to mind is that what should be good to be chosen as key for the items in lists. It is recommended to use a string as a key that uniquely identifies the items in the list. Below example shows a list with string keys:  
Javascript
const numbers = [ 1, 2, 3, 4, 5 ];  const updatedNums = numbers.map((number)=>{return <li key={index}>{number} </li>;});
You can also assign the array indexes as keys to the list items. The below example assigns array indexes as key to the elements. 
Javascript
const numbers = [ 1, 2, 3, 4, 5 ];  const updatedNums = numbers.map((number, index)=><li key = {index}>{number} </li>);
Assigning indexes as keys are highly discouraged because if the elements of the arrays get reordered in the future then it will get confusing for the developer as the keys for the elements will also change. 
Using Keys with Components
Consider a situation where you have created a separate component for list items and you are extracting list items from that component. In that case, you will have to assign keys to the component you are returning from the iterator and not to the list items. That is you should assign keys to <Component /> and not to <li> A good practice to avoid mistakes is to keep in mind that anything you are returning from inside of the map() function is needed to be assigned key. 
Below code shows incorrect usage of keys: 
Javascript
import React from 'react';import ReactDOM from 'react-dom';// Component to be extractedfunction MenuItems(props){const item = props.item;return(<li key = {item.toString()}>{item}</li> );}           // Component that will return an// unordered listfunction Navmenu(props){const list = props.menuitems;const updatedList = list.map((listItems)=>{return (<MenuItems item = { listItems } /> ); });           return(<ul>{updatedList}</ul>);} const menuItems = [1, 2, 3, 4, 5]; ReactDOM.render(<Navmenu menuitems = {menuItems} />, document.getElementById('root'));         
Output: 
You can see in the above output that the list is rendered successfully but a warning is thrown to the console that the elements inside the iterator are not assigned keys. This is because we had not assigned key to the elements we are returning to the map() iterator.
Below example shows correct usage of keys: 
Javascript
import React from "react";import ReactDOM from "react-dom";// Component to be extractedfunction MenuItems(props) {    const item = props.item;    return <li>{item}</li>;}  // Component that will return an// unordered listfunction Navmenu(props) {    const list = props.menuitems;    const updatedList = list.map((listItems) => {        return <MenuItems key={listItems.toString()} item={listItems} />;    });      return <ul>{updatedList}</ul>;}  const menuItems = [1, 2, 3, 4, 5];  ReactDOM.render(    <Navmenu menuitems={menuItems} />,    document.getElementById("root"));
The above code will run successfully without any warning message. 
Uniqueness of Keys
We have told many times while discussing about keys that keys assigned to the array elements must be unique. By this, we did not mean that the keys should be globally unique. All the elements in a particular array should have unique keys. That is, two different arrays can have the same set of keys.In the below code we have created two different arrays menuItems1 and menuItems2. You can see in the below code that the keys for the first 5 items for both arrays are the same still the code runs successfully without any warning. 
Javascript
import React from "react";import ReactDOM from "react-dom";// Component to be extractedfunction MenuItems(props) {    const item = props.item;    return <li>{item}</li>;}  // Component that will return an// unordered listfunction Navmenu(props) {    const list = props.menuitems;    const updatedList = list.map((listItems) => {        return <MenuItems key={listItems.toString()} item={listItems} />;    });      return <ul>{updatedList}</ul>;}  const menuItems1 = [1, 2, 3, 4, 5];const menuItems2 = [1, 2, 3, 4, 5, 6];  ReactDOM.render(    <div>        <Navmenu menuitems={menuItems1} />        <Navmenu menuitems={menuItems2} />    </div>,    document.getElementById("root"));
Output:
Note: Keys are not the same as props, only the method of assigning “key” to a component is the same as that of props. Keys are internal to React and can not be accessed from inside of the component like props. Therefore, we can use the same value we have assigned to the Key for any other prop we are passing to the Component. 
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