ReactJS - Components and Props

ReactJS - Components and Props

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Components let you split the UI into independent, reusable pieces, and think about each piece in isolation.

Conceptually, components are like JavaScript functions. They accept arbitrary inputs (called props) and return React elements describing what should appear on the screen.

Function and Class Components

The simplest way to define a component is to write a JavaScript function:

function Welcome(props) {
  return <h1>Hello, {}</h1>;

This function is a valid React component because it accepts a single props (which stands for properties) object argument with data and returns a React element. We call such components function components because they are literally JavaScript functions.

You can also use an ES6 class to define a component:

class Welcome extends React.Component {
  render() {
    return <h1>Hello, {}</h1>;

The above two components are equivalent from Reacts point of view.

Classes have some additional features that we will discuss in the next sections. Until then, we will use function components for their conciseness.

Props are Read-Only

Whether you declare a component as a function or a class, it must never modify its own props. Consider this sum function:

function sum(a, b) {
  return a + b;

Such functions are called pure because they do not attempt to change their inputs, and always return the same result for the same inputs.

In contrast, this function is impure because it changes its own input:

function withdraw(account, amount) { -= amount;

React is pretty flexible but it has a single strict rule:

All React components must act like pure functions with respect to their props.

Of course, application UIs are dynamic and change over time. In the next section,we will introduce a new concept of state. State allows React components to change their output over time in response to user actions, network responses, and anything else, without violating this rule.


Last update on 20 Jan 2020