React Props

How to use props to pass data around your React components

Props is how Components get their properties. Starting from the top component, every child component gets its props from the parent. In a function component, props is all it gets passed, and they are available by adding props as the function argument:

const BlogPostExcerpt = props => {
  return (
    <div>
      <h1>{props.title}</h1>
      <p>{props.description}</p>
    </div>
  )
}

In a class component, props are passed by default. There is no need to add anything special, and they are accessible as this.props in a Component instance.

import React, { Component } from 'react'

class BlogPostExcerpt extends Component {
  render() {
    return (
      <div>
        <h1>{this.props.title}</h1>
        <p>{this.props.description}</p>
      </div>
    )
  }
}

Passing props down to child components is a great way to pass values around in your application. A component either holds data (has state) or receives data through its props.

It gets complicated when:

  • you need to access the state of a component from a child that’s several levels down (all the previous children need to act as a pass-through, even if they do not need to know the state, complicating things)
  • you need to access the state of a component from a completely unrelated component.

Default values for props

If any value is not required we need to specify a default value for it if it’s missing when the Component is initialized.

BlogPostExcerpt.propTypes = {
  title: PropTypes.string,
  description: PropTypes.string
}

BlogPostExcerpt.defaultProps = {
  title: '',
  description: ''
}

Some tooling like ESLint have the ability to enforce defining the defaultProps for a Component with some propTypes not explicitly required.

How props are passed

When initializing a component, pass the props in a way similar to HTML attributes:

const desc = 'A description'
//...
<BlogPostExcerpt title="A blog post" description={desc} />

We passed the title as a plain string (something we can only do with strings!), and description as a variable.

Children

A special prop is children. That contains the value of anything that is passed in the body of the component, for example:

<BlogPostExcerpt title="A blog post" description="{desc}">
  Something
</BlogPostExcerpt>

In this case, inside BlogPostExcerpt we could access “Something” by looking up this.props.children.

While Props allow a Component to receive properties from its parent, to be “instructed” to print some data for example, state allows a component to take on life itself, and be independent of the surrounding environment.

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