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Props JS: React

The Card component we wrote earlier is useless in practice because it doesn't allow you to change texts. And it's not a good idea to create your own component for each specific Card block. Not to mention the fact that most often this is simply impossible, because the data is substituted dynamically.

You can send data to components, and this is done using props:

See the Pen js_react_props by Hexlet (@hexlet) on CodePen.

As you can see, from the outside props are passed as attributes in HTML, which are accessible inside the component from the props object. And this transfer of input data shouldn't be new to you anymore. Built-in components take props, such as className and others, as input in the same way.

const vdom = (
  <div className="row">
    <div className="col-6">
      <HelloMessage name="Kate" />
    <div className="col-6">
      <HelloMessage name="Mark" />

Props are a very simple mechanism for transferring data to components, and it usually doesn't cause any difficulties. The main thing to remember when working with props: is that they can't be changed. Primarily because of the way React works, it simply won't get you anywhere, and second, React has an entirely different mechanism for dealing with changeable state, which we'll look at later.

Spread operator

When working with props, it's often necessary to pass many parameters, or these parameters are present in the code as an object. In this case, you can simplify the transfer by using a spread operator.

const params = {
  className: 'row',
  title: 'name',
const name = 'Eva';
const vdom = <div id="container" {...params}>
  Hello, {name}

The code above is equivalent to the following example:

const name = 'Eva';
const vdom = <div id="container" className="row" title="name">
  Hello, {name}

Default Props

Another challenge developers face is setting default values for props (for cases where some props are not passed). It is easiest to set them right inside the render function using this approach:

const title = this.props.title || 'hi!';

This will work, but could potentially lead to performance problems, as well as other issues. The topic of productivity will be covered in one of the last lessons.

React provides a way to set default props values. Example:

class Header extends React.Component {
  render() {
    const { text } = this.props;
    return (

Header.defaultProps = {
  text: 'Hello, world!',

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