In React, keys are special attributes used when rendering arrays of elements, such as when mapping over a list of items to generate a list of React components. The key prop serves as a unique identifier for each element in the list and helps React efficiently update the UI when the list changes.
The role of keys in React is twofold:
Efficient Updates: When React renders a list of components, it needs a way to identify each element uniquely. By providing a key prop to each component, React can keep track of which elements have changed, been added, or been removed. This helps React optimize the rendering process and reduce unnecessary re-renders. Without keys, React would have to re-render the entire list every time the list changes, which can negatively impact performance and user experience.
Reconciliation: React uses a process called "reconciliation" to determine how to update the DOM efficiently when the state or props of a component change. When the list of children changes, React performs a diffing algorithm to compare the new list of elements with the previous one. Keys play a crucial role in this process by allowing React to match elements with their corresponding DOM nodes. This helps React identify which elements need to be added, removed, or updated in the DOM, minimizing unnecessary DOM manipulations and improving performance.
It's important to note that keys must be unique among siblings but not necessarily globally unique across the entire application. For example, when rendering a list of items fetched from a database, using a unique identifier from the database as the key is typically sufficient.
Example of using keys in rendering a list:
import React from 'react'; const TodoList = ({ todos }) => { return ( <ul> {todos.map((todo) => ( <li key={todo.id}>{todo.text}</li> ))} </ul> ); }; export default TodoList; In this example, the todos prop is an array of objects, each representing a todo item with properties id and text. By using key={todo.id}, we ensure that each todo item is uniquely identified by its id, allowing React to efficiently update the list when the todos array changes.