- Use props to make reusable component templates
- Understand how props are passed to a component
We'll take the next step with React components and examine how they can be used as dynamic templates by using props.
As the building blocks of React applications, components are dynamic, in that they can describe a template of HTML and fill in variable data. This lesson builds a real example of a blogging application to illustrate dynamic components.
We will use the following components:
BlogContent
- contains the content of the blog postComment
- contains one user's commentBlogPost
- the 'top level' React component, which is responsible for rendering bothBlogContent
andComment
Time to put the dynamic aspect of components to use! Let's start with the
BlogContent
component. The following snippet shows how we can describe
variables inside a component:
function BlogContent(props) {
return <div>{props.articleText}</div>;
}
You should see something new in the above code. Our function has a parameter
defined called props
. Also, inside the return statement, we have this funky
syntax: {props.articleText}
.
This line is telling React to place the value that props.articleText
represents within the <div>
. Ok, so where does props.articleText
come from?
React allows us to pass units of information from a parent component down to a
child component. We call these props, which we will dig more into in a
later lesson. Let's see how we can pass information from BlogPost
down to its
child BlogContent
:
function BlogPost() {
return (
<div>
<BlogContent articleText="Dear Reader: Bjarne Stroustrup has the perfect lecture oration." />
</div>
);
}
In the above, we see that when we render the BlogContent
component, we also
create a prop called articleText
that we assign a value of "Dear Reader:
Bjarne Stroustrup has the perfect lecture oration." This value is accessible
from within the BlogContent
component as props.articleText
!
To create props, we write them the same way as writing attributes for an HTML
tag. For example, to assign a <div>
an id, we give it an attribute:
<div id="card">Hello!</div>
To assign a prop to a component, we use the same syntax:
function ParentComponent() {
// passing prop to ChildComponent
return <ChildComponent text="Hello!" number={2} />;
}
function ChildComponent(props) {
// using the values of the text and number props
return (
<div>
{props.text} {props.number}
</div>
);
}
But remember, this is JSX and not HTML!
One more thing about props: they can be any data type! In our example, we pass a string as a prop. But we can pass a number, boolean, object, function, etc. as a prop!
Let's expand a bit on props here. Taking a look at both of our components will give us a better understanding of how data can be passed from one component to another:
// BlogPost.js
// PARENT COMPONENT
function BlogPost() {
return (
<div>
{/* BlogContent is being returned from BlogPost */}
{/* Therefore, BlogContent a child of BlogPost */}
<BlogContent articleText="Dear Reader: Bjarne Stroustrup has the perfect lecture oration." />
</div>
);
}
// BlogContent.js
// CHILD COMPONENT
function BlogContent(props) {
return <div>{props.articleText}</div>;
}
When one component returns another component, this creates a special relationship between these two components. The component being returned is the child component, and the component returning that child is the parent component.
The only way for a parent component to pass data to its child components is via props.
On this line:
// BlogPost.js
<BlogContent articleText="Dear Reader: Bjarne Stroustrup has the perfect lecture oration." />
We are adding a prop of articleText
to our BlogContent
component.
If we add a console.log
in the BlogContent
component to inspect the props:
// BlogContent.js
function BlogContent(props) {
console.log(props);
return <div>{props.articleText}</div>;
}
We'll see an object with key-value pairs related to the data we passed down from the parent component!
// BlogContent.js
console.log(props);
// => { articleText: "Dear Reader: Bjarne Stroustrup has the perfect lecture oration." }
We can add as many additional props as we want, by assigning them in the parent component:
<BlogContent
articleText="Dear Reader: Bjarne Stroustrup has the perfect lecture oration."
isPublished={true}
minutesToRead={1}
/>
Note: for props that are strings, we don't need to place curly braces around the values; for other data types (numbers, booleans, objects, etc), we need curly braces.
And all of these props will be added as keys on the props object in the child component:
// BlogContent.js
console.log(props);
/*
{
articleText: "Dear Reader: Bjarne Stroustrup has the perfect lecture oration.",
isPublished: true,
minutesToRead: 1
}
*/
Having the ability to share this data between components by passing props down
to a child component from a parent makes our components incredibly flexible! For
example, here's how we could expand our BlogContent
component based on those
additional props:
function BlogContent(props) {
console.log(props);
if (!props.isPublisbed) {
// hide unpublished content
// return null means "don't display any DOM elements here"
return null;
} else {
// show published content
return (
<div>
<h1>{props.articleText}</h1>
<p>{props.minutesToRead} minutes to read</p>
</div>
);
}
}
We still need a Comment
component that we can use for each comment in a
BlogPost
. The Comment
component would look something like:
function Comment(props) {
return <div>{props.commentText}</div>;
}
This component, when used, will display content that is passed down to it,
allowing us to pass different content to multiple Comment
components. Let's
add them in. Of course, with components being re-usable, we can make as many as
we want:
function BlogPost() {
return (
<div>
<BlogContent articleText="Dear Reader: Bjarne Stroustrup has the perfect lecture oration." />
<Comment />
<Comment />
<Comment />
</div>
);
}
...and just as before, we can pass content data down to them:
function BlogPost() {
return (
<div>
<BlogContent articleText="Dear Reader: Bjarne Stroustrup has the perfect lecture oration." />
<Comment commentText="I agree with this statement. - Angela Merkel" />
<Comment commentText="A universal truth. - Noam Chomsky" />
<Comment commentText="Truth is singular. Its ‘versions’ are mistruths. - Sonmi-451" />
</div>
);
}
There is quite a bit going on here. Most notably, we are passing information
from a parent component to many child components. Specifically, we are doing
this by creating a prop called commentText
to pass to each Comment
component, which is then accessible in each instance of Comment
as
props.commentText
. Let's expand the HTML that this would ultimately render:
<div>
<div>Dear Reader: Bjarne Stroustrup has the perfect lecture oration.</div>
<div>I agree with this statement. - Angela Merkel</div>
<div>A universal truth. - Noam Chomsky</div>
<div>Truth is singular. Its ‘versions’ are mistruths - Sonmi-451</div>
</div>
...but seeing is believing so let's look at this in technicolor! Following is an inspection of the real live DOM elements that React rendered when we blasted this code into a new application (classes, IDs, and minor CSS have been added for a better visual display):
Alright now! Take a moment. Stretch your limbs, make a sandwich, let the glorious paradigm sink in. Dynamic components are a core facet of React programming, and most of what we do as React programmers builds upon them.
While HTML elements are the basic building blocks of a website, (for example, a
<div>
), a React application usually consists of several React components
combined together. Unlike simple HTML elements, React components are smarter and
bigger. They allow you to do much more and incorporate logic into how content
displays.
Components:
- are modular, reusable, and enable a 'templating' functionality
- help us organize our user interface's logic and presentation
- enable us to think about each piece in isolation, enabling us to apply structure to complex programs
Props:
- are passed from a parent component to a child component
- can be accessed in the child components via an object that is passed into our component function
- can hold any kind of data (strings, numbers, booleans, objects, even functions!)
Going forward we will expand on what we can do with components, how they fit into the larger React landscape, and what built-in functionality they come with.