Your components will often need to display different things depending on different conditions. In React, you can conditionally render JSX using JavaScript syntax like if
statements, &&
, and ? :
operators.
Lets say you have a PackingList
component rendering several Item
s which can be marked as packed or not:
function Item({ name, isPacked }) { return <li className="item">{name}</li>; } export default function PackingList() { return ( <section> <h1>Sally Ride's Packing List</h1> <ul> <Item isPacked={true} name="Space suit" /> <Item isPacked={true} name="Helmet with a golden leaf" /> <Item isPacked={false} name="Photo of Tam" /> </ul> </section> ); }
Notice that some of the Item
components have their isPacked
props set to true
instead of false
. You want to add a checkmark(✅) to packed items if isPacked={true}
.
You can write this as an if/else statement like so:
if (isPacked) { return <li className="item">{name} ✔</li>; } return <li className="item">{name}</li>;
If the isPacked
prop is true
, this code returns a different JSX tree. With this change, some of the items get a checkmark at the end:
function Item({ name, isPacked }) { if (isPacked) { return <li className="item">{name} ✔</li>; } return <li className="item">{name}</li>; } export default function PackingList() { return ( <section> <h1>Sally Ride's Packing List</h1> <ul> <Item isPacked={true} name="Space suit" /> <Item isPacked={true} name="Helmet with a golden leaf" /> <Item isPacked={false} name="Photo of Tam" /> </ul> </section> ); }
Notice how you’re creating branching logic with JavaScript’s if
and return
statements. In React, control flow (like conditions) is handled by JavaScript.
null
In some situations, you won't want to render anything at all. For example, say you don't want to show packed items at all. A component must return something. In this case, you can return null
:
if (isPacked) { return null; } return <li className="item">{name}</li>;
If isPacked
is true, the component will return nothing, null
. Otherwise, it will return JSX to render.
function Item({ name, isPacked }) { if (isPacked) { return null; } return <li className="item">{name}</li>; } export default function PackingList() { return ( <section> <h1>Sally Ride's Packing List</h1> <ul> <Item isPacked={true} name="Space suit" /> <Item isPacked={true} name="Helmet with a golden leaf" /> <Item isPacked={false} name="Photo of Tam" /> </ul> </section> ); }
In practice, returning null
from a component isn’t common because it might surprise a developer trying to render it. More often, you would conditionally include or exclude the component in the parent component’s JSX. Here’s how to do that!
In the previous example, you controlled which (if any!) JSX tree would be returned by the component. You may already have noticed some duplication in the render output:
<li className="item">{name} ✔</li>
is very similar to
<li className="item">{name}</li>
Both of the conditional branches return <li className="item">...</li>
if (isPacked) { return <li className="item">{name} ✔</li>; } return <li className="item">{name}</li>;
While this duplication isn’t harmful, it could make your code harder to maintain. What if you want to change the className
? You’d have to do it in two places in your code! In such a situation, you could conditionally include a little JSX to make your code more DRY.
? :
)JavaScript has a compact syntax for writing a conditional expression — the conditional operator or “ternary operator”.
Instead of this:
if (isPacked) { return <li className="item">{name} ✔</li>; } return <li className="item">{name}</li>;
You can write this:
return ( <li className="item"> {isPacked ? name + ' ✔' : name} </li> );
You can read it as “if isPacked
is true, then (?
) render name + ' ✅'
, otherwise (:
) render name
”.
Now let’s say you want to wrap the completed item’s text into another HTML tag, like <del>
to strike it out. You can add even more newlines and parentheses so that it’s easier to nest more JSX in each of the cases:
function Item({ name, isPacked }) { return ( <li className="item"> {isPacked ? ( <del> {name + ' ✔'} </del> ) : ( name )} </li> ); } export default function PackingList() { return ( <section> <h1>Sally Ride's Packing List</h1> <ul> <Item isPacked={true} name="Space suit" /> <Item isPacked={true} name="Helmet with a golden leaf" /> <Item isPacked={false} name="Photo of Tam" /> </ul> </section> ); }
This style works well for simple conditions, but use it in moderation. If your components get messy with too much nested conditional markup, consider extracting child components to clean things up. In React, markup is a part of your code, so you can use tools like variables and functions to tidy up complex expressions.
&&
)Another common shortcut you’ll encounter is the JavaScript logical AND (&&) operator. Inside React components, it often comes up when you want to render some JSX when the condition is true, or render nothing otherwise. With &&, you could conditionally render the checkmark only if isPacked is true:
return ( <li className="item"> {name} {isPacked && '✔'} </li> );
You can read this as “if isPacked
, then (&&
) render the checkmark, otherwise, render nothing”.
Here it is in action:
function Item({ name, isPacked }) { return ( <li className="item"> {name} {isPacked && '✔'} </li> ); } export default function PackingList() { return ( <section> <h1>Sally Ride's Packing List</h1> <ul> <Item isPacked={true} name="Space suit" /> <Item isPacked={true} name="Helmet with a golden leaf" /> <Item isPacked={false} name="Photo of Tam" /> </ul> </section> ); }
A JavaScript && expression returns the value of its right side (in our case, the checkmark) if the left side (our condition) is true
. But if the condition is false
, the whole expression becomes false
. React considers false
as a "hole" in the JSX tree, just like null
or undefined
, and doesn't render anything in its place.
Don't put numbers on the left side of &&
.
To test the condition, JavaScript converts the left side to a boolean automatically. However, if the left side is 0
, then the whole expression gets that value (0
), and React will happily render 0
rather than nothing.
For example, a common mistake is to write code like messageCount && <p>New messages</p>
. It's easy to assume that it renders nothing when messageCount
is 0
, but it really renders the 0
itself!
To fix it, make the left side a boolean: messageCount > 0 && <p>New messages</p>
.
When the shortcuts get in the way of writing plain code, try using an if
statement and a variable. You can reassign variables defined with let
, so start by providing the default content you want to display, the name:
let itemContent = name;
Use an if
statement to reassign a JSX expression to itemContent
if isPacked
is true
:
if (isPacked) { itemContent = name + " ✔"; }
Curly braces open the "window into JavaScript". Embed the variable with curly braces in the returned JSX tree, nesting the previously calculated expression inside of JSX:
<li className="item"> {itemContent} </li>
This style is the most verbose, but it's also the most flexible. Here it is in action:
function Item({ name, isPacked }) { let itemContent = name; if (isPacked) { itemContent = name + " ✔"; } return ( <li className="item"> {itemContent} </li> ); } export default function PackingList() { return ( <section> <h1>Sally Ride's Packing List</h1> <ul> <Item isPacked={true} name="Space suit" /> <Item isPacked={true} name="Helmet with a golden leaf" /> <Item isPacked={false} name="Photo of Tam" /> </ul> </section> ); }
Like before, this works not only for text, but for arbitrary JSX too:
function Item({ name, isPacked }) { let itemContent = name; if (isPacked) { itemContent = ( <del> {name + " ✔"} </del> ); } return ( <li className="item"> {itemContent} </li> ); } export default function PackingList() { return ( <section> <h1>Sally Ride's Packing List</h1> <ul> <Item isPacked={true} name="Space suit" /> <Item isPacked={true} name="Helmet with a golden leaf" /> <Item isPacked={false} name="Photo of Tam" /> </ul> </section> ); }
If you're not familiar with JavaScript, this variety of styles might seem overwhelming at first. However, learning them will help you read and write any JavaScript code -- and not just React components! Pick the one you prefer for a start, and then consult this reference again if you forget how the other ones work.
Make a component in src/components/show-icon.jsx
which is default exported, It will accept title(string)
and completed(boolean)
as props, return <title> ❌
if completed={false}
and <title> ✅
if completed={true}
in li
.
Make a component in src/components/importance.jsx
which is default exported. It will accept title(string)
and importance(number)
as props, return a li
with className={"red-text"}
if importance
is more than 5 and return li
with className={""}
if importance
is less than or equal to 5.
Make a component in src/components/result.jsx
which is default exported. It will accept subject(string)
and failed(boolean)
as props, return a li
with <subject>: passed
is failed={false}
and return null
if failed={true}
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