ReactJS | Keys

In the previous article on ReactJS | Lists, we had discussed about 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 in 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 indentity to the elements in the lists. The next thing that comes in mind is that what should be good to be choose 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:

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const numbers = [ 1, 2, 3, 4, 5 ];
  
const updatedNums = numbers.map((number)=>{
return <li>{ number } </li>;
});

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You can also assign the array indexes as key to the list items. Below example assigns array indexes as key to the elements.



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const numbers = [ 1, 2, 3, 4, 5 ];
  
const updatedNums = numbers.map((number, index)=>{
return <li>{ number } </li>;
});

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Assigning indexes as keys is highly discouraged because if the elements of the arrays get’s reordered in future then it will get confusing for the developer as the key’s 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 mistake is to keep in mind that anything you are returning from inside of map() function is needed to be assigned key.

Below code shows incorrect usage of keys:

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import React from 'react';
import ReactDOM from 'react-dom';
  
// Component to be extracted
function MenuItems(props)
{
    const item = props.item;
  
        return(
            <li>
                {item}
            </li>    
        );
}
   
// Component that will return an
// unordered list
function Navmenu(props)
{
        const list = props.menuitems;
  
    const updatedList = list.map((listItems)=>{
            return (
                  
        );
    });
   
    return(
        <ul>{updatedList}</ul>);
}
   
const menuItems = [1, 2, 3, 4, 5];
   
ReactDOM.render(
    
    document.getElementById('root')
);

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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:

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import React from 'react';
import ReactDOM from 'react-dom';
  
// Component to be extracted
function MenuItems(props)
{
    const item = props.item;
  
        return(
            <li>
                {item}
            </li>    
        );
}
   
// Component that will return an
// unordered list
function Navmenu(props)
{
        const list = props.menuitems;
  
    const updatedList = list.map((listItems)=>{
            return (
                  
        );
    });
   
    return(
        <ul>{updatedList}</ul>);
}
   
const menuItems = [1, 2, 3, 4, 5];
   
ReactDOM.render(
    
    document.getElementById('root')
);

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The above code will run successfully without any warning message.

Uniqueness of Keys

We have told many times while dicussing about keys that keys assign to the array elements must be unique. By this we did not meant 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 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 same still the code runs succesfully without any warning.

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import React from 'react';
import ReactDOM from 'react-dom';
  
// Component to be extracted
function MenuItems(props)
{
    const item = props.item;
  
        return(
            <li>
                {item}
            </li>    
        );
}
   
// Component that will return an
// unordered list
function Navmenu(props)
{
        const list = props.menuitems;
  
    const updatedList = list.map((listItems)=>{
            return (
                  
        );
    });
   
    return(
        <ul>{updatedList}</ul>);
}
   
const menuItems1 = [1, 2, 3, 4, 5];
const menuItems2 = [1, 2, 3, 4, 5, 6];
   
ReactDOM.render(
    <div>
          
          
    </div>, 
    document.getElementById('root')
);

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Note: Keys are not same as props, only the method of assigning “key” to a component is 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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