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Hey guys in this lesson we're going to level up our knowledge around React components and to do that
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we're going to learn about something called props. So head over to the starting code sandbox and go ahead
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and fork it. And once you have it
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feel free to download it to use in your local environment or follow along with me in code sandbox.
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So here I've got a really simple contacts app. So I've got three people that I supposedly know their names,
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a picture of them that I found on the Internet, some fake phone numbers and fake emails. As you can see
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if we look inside our inde.js, this is pretty repetitive and it's also very long. And this is
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one of the perfect use cases for React components because we have these repetitive elements that is
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cluttering up our code making it very long and very complex unnecessarily.
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So let's go ahead and extract each of these contacts as a brand new component.
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I'm going to create it just inside my index.js so you can see quite easily
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both the component and the rendering side by side.
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And I'm going to call this component a new card like a contact card, and it's going to return
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each of these contact items.
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So I'm just going to cut Beyonce out of there and I'm going to insert her into a div.
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So now this h2 image and two paragraph elements are now being returned from my card function.
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Now I'm actually just gonna go ahead and take out these other guys from my render function and I'm gonna
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keep my website as simple as I can
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so you can see easily what's happening.
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So instead of having all of those divs, I can now create maybe three cards instead
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right?
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So I've got three cards like so, and once it reloads you can see that yes I do have three cards which
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all have an h2 image and two paragraphs but they're all of Beyonce.
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So that's not what I want.
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Instead I want to have different pieces of information in each of these reusable card components. With
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the knowledge that we've gotten so far,
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it's actually very difficult to make these components reusable. And it's because we haven't got any ability
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to send these card components some custom data so that each of the cards gets rendered differently.
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Now let's think about how this problem is solved in plain old HTML.
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So if we had over to the index.html and let's just go ahead and delete all of our cards from here so
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that we clear up our screen. And right below my root div where we know our React components are going
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to be rendered,
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I'm gonna go ahead and create just a plain HTML element. And remember that when you're creating a React
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app you're not limited to just using React.
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You can just insert it any way you like using a div with a particular id. And the rest of the file
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can still be worked like a regular JavaScript or HTML file.
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So that means if you have an existing website, it's really easy to just insert a root div and then start
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adding React code into that particular position.
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So I'm going to create a plain HTML element which is going to be an input element and I'm going to
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make itself closing as well.
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And you can see that it shows up in here as a little text box. Now this input element has attributes
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right? Attributes which I can set with custom values.
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For example, I could change the id of this input.
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Let's call it fName
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so like a field to input your first name, and then I can also give it a placeholder.
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So I could say that enter your first name.
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And once we refresh you can see the place holder goes up in gray.
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And I can even set the value even before the users had a chance to interact with it.
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So let's just say I'm going to put my own name in there and you can see that I've now got an input field
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with a value that's already pre-existing inside.
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And if I delete it and make it empty, then you'll get to see my placeholder text.
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So this is kind of a way for us to pass custom pieces information via these attributes to a particular
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HTML element
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right?
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And if we go ahead and take a look at this app in a separate tab and I open up my Chrome developer tools
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then I can actually access this element by just grabbing hold of document.getElementById and we
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call it fName.
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So this should actually get hold of this particular input element.
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And I'm going to save it into a variable. I'll just call it input and then hit enter.
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So now I've got this input value stored then I can actually access all of its values.
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I can say input.placeholder to print out the value of the placeholder attribute.
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I can also say something like input.value to print out the current value of the inputs and I can even
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change this right?
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So effectively we're using these attributes from our HTML elements to use and custom pieces of data to
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that particular element.
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And over here when we tap into that element using get element by id and we have a reference to it, then
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you can see it's almost like as if we have the input as an object and we're accessing each of its properties
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like .placeholder or .value.
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Now these attributes are predetermined and there are lists of them in the MDN docs but we can't
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go beyond what is actually here.
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We can't define our own custom attribute
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right?
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If I created something called, I don't know, just angela and I tried to give it a value.
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And I go into my React app and I try to tap into input.angela, that doesn't actually work. But if
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you think about your React components almost as a custom HTML element then we can define these attributes.
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And in the React component world, those attributes are called properties and you'll usually hear them
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referred to as props. Let's say that I decided to create a new card component from my card function. And
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what happens behind the scenes is it's almost like I'm calling this function card and it returns a custom
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HTML element based on my instructions. But when we take a look at these custom React components, we can
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pass properties using very similar syntax to what you saw over here.
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So if I go ahead and cut this input out of my index.html and I paste it down here just below my
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card, you can see that with our card component we can do exactly the same thing.
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So I can pass my card a value, for example name, and I can set that equal to Beyonce for example. And
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when this card component gets created, it will have a property called name that's set to equal to Beyonce
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And we can get hold of it by adding a name to the inputs to access those things that are sent over.
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And it's customary to call this props. For example inside this function I can console log the value of
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props and if I hit save and we take a look in our console, you can see that it receives an object.
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And currently the object just has one property with a value set to Beyonce.
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So what this means is that in fact, we can actually completely remove all of these hard coded parts of
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our React Components and instead rely on inserting the values of these props.
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So I could say props.name which I know to be the property that I sent over and it's equal to 
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Beyonce.
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And now what I can do is I can for example copy the source and add that as a property. So I'll create
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a new property called image and set it to equal the URL of that image. I will create a property called
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tel for telephone and I'll set it equal to that number. And I'll also create a property called
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email which I'll set equal to that made up email.
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So now inside our card component I can access that object called props which has all of these properties
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passed in as an object and I can tap into each of these properties as long as I know what they are called.
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So the source was stored inside a property called img, so I can write props.img.
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But remember that this is Javascript and we're inside an beyondelement inside our JSX file.
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So be sure to enclose it in a set of curly braces if you want it to be interpreted as Javascript.
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Next I've got my props.
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tel and my props.email.
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So I've ended up with exactly the same card as before
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but the crucial difference is that this card no longer contains any Beyonce specific information.
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And what I can do now is I can create as many of these cards as I need with different pieces of information.
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And I want to pose this as a challenge to you. Go ahead and try to create another card with an imaginary
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contact if you wish or a real contact even, and see if you can add a second card to our website.
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That second card should appear just below Beyonce but it should have a different name, a different
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image, email and phone number. Pause the video now and see if you can successfully reuse our custom card
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component.
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All right.
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So I'm going to create another card and this is a self closing tag with no children.
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And I'm going to add some properties.
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And remember that because inside our card function it's looking for these properties name, image,
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tel, email. We have to keep it consistent in between the cards.
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So let's say that name is Jack Bauer.
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Img is equal to some sort of image that I find online.
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And then we've got tel which will set equal to some random numbers and email we'll set equal to
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so perfect.
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We've now got two cards with completely different information and we're able to reuse this card component
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with all of its structure as well as styling if you wish.
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So these could actually have class names associated with them or inline styling if you wish
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but all we need to do here in our render function is to provide the customized pieces of data and everything
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that stays the same lives inside this card component.
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So now as you can imagine, if you wanted a contact list with many many people, you can simply just reuse
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your card component.
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And this brings us to a good point.
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If you remember previously, I said that if you had a style let's say let's just give it a class name
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let's just call it my style, and all it does is changes the text color to red and we save that CSS file
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and we go back into our React code,
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you can see that if I was to add that class to my div for example.
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So I'll say className = "my-style" and I hit save and refresh my websites,
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all the text inside that div has been changed to red right? But I can't do the same thing inside my custom
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card component.
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I can't for example add the class name attribute and then apply my-style. Even if I refresh a million
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times
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it's still not going to work.
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And this is because inside our custom components, it sees each of these as a custom property.
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So it thinks that you're making up a name called className and you would like to access it inside this
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card component under the props object instead of seeing it as an HTML attribute.
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So whenever you're using HTML predetermined attributes, they always have to go into the HTML elements which
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are highlighted in different color and also they're in lower case.
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So this is the reason why we can't do that. But inside our custom components, we can create any name we
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want and associate it with any value we need.
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So it's almost like we're calling this card function and passing over an object through the input called
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props.
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And then inside the function we, use those inputs to customize this HTML element and then we return
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it and output it into our DOM to be seen on the website. In summary with HTML attributes,
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we have the HTML tag and then we have a space and we get to use these predetermined fixed name attributes
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such as id or place holder or value.
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And these of course depend on the actual element that we're using.
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And when we use these attributes, we can pass in some custom data. And that data can then be accessed
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on the element by tapping into say the .id or .placeholder or .value properties.
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Now when it comes to React, what we have is a very similar structure.
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So we have our custom components and we can create as many of these properties or props as we wish, we
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can name them anything we like.
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They're not fixed.
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Nobody has defined them ahead of time.
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In fact you are the one that's creating this custom card element and you get to specify which attributes
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or properties they should have.
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But these values for each of these properties then gets passed along when this card component is created.
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And when we output this HTML element we can incorporate those properties into the creation of that element.
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And it's usually via an input called props.
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So the easiest way to imagine it is just simply that you're creating a new Javascript object with these
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key and value pairs and then that whole object gets sent over as the props object and then you can tap
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into it using .name, .telephone,
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as long as they match up with the names in your component element. In the next lesson, we're going to
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test out your understanding of props and you're going to be using everything you learn here to complete
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a challenge and build out a fully fledged contacts app. So for all of that and more, I'll see you there.