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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.