In React, you can create distinct components that encapsulate behavior you need. Then, you can render only some of them, depending on the state of your application.
Conditional rendering in React works the same way conditions work in JavaScript. Use JavaScript operators like if or the conditional operator to create elements representing the current state, and let React update the UI to match them.
First, let’s review how you transform lists in JavaScript.
Given the code below, we use the map() function to take an array of numbers and double their values. We assign the new array returned by map() to the variable doubled and log it:
const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map((number) => number * 2);
console.log(doubled);
Keys help React identify which items have changed, are added, or are removed. Keys should be given to the elements inside the array to give the elements a stable identity:
const numbers = [1, 2, 3, 4, 5];
const listItems = numbers.map((number) =>
<li key={number.toString()}>
{number}
</li>
);
The best way to pick a key is to use a string that uniquely identifies a list item among its siblings. Most often you would use IDs from your data as keys:
const todoItems = todos.map((todo) =>
<li key={todo.id}>
{todo.text}
</li>
);
When you don’t have stable IDs for rendered items, you may use the item index as a key as a last resort:
const todoItems = todos.map((todo, index) =>
// Only do this if items have no stable IDs
<li key={index}>
{todo.text}
</li>
);
HTML form elements work a bit differently from other DOM elements in React, because form elements naturally keep some internal state. For example, this form in plain HTML accepts a single name:
This form has the default HTML form behavior of browsing to a new page when the user submits the form. If you want this behavior in React, it just works. But in most cases, it’s convenient to have a JavaScript function that handles the submission of the form and has access to the data that the user entered into the form. The standard way to achieve this is with a technique called “controlled components”.