How to use useState hook to set markup text? - reactjs

Question

I have a functional component with the following hook:-
const [imagePreview, setImagePreview] = useState('');
Later on, the change event, I have this:-
if (imagePreviewUrl) {
setImagePreview( (<div>
<br/> <img src={imagePreviewUrl} alt="icon" width="200" />
</div>));
}
Looks like it doesn't set anything. how do I set it? I could do it as:-
let imagePreview = null;
if (imagePreviewUrl) {
imagePreview = (<div>
<br/> <img src={imagePreviewUrl} alt="icon" width="200" />
</div>);
}
But it is bit ugly. I think useState is the recommended way to do it.
Update with more code:-
const fileChangedHandler = event => {
setSelectedFile(event.target.files[0]);
let reader = new FileReader();
reader.onloadend = () => {
setImagePreviewUrl(reader.result);
}
reader.readAsDataURL(event.target.files[0])
if (imagePreviewUrl) {
// can't break into multiline, sytax error
setImagePreview('(<div><br/> <img src={imagePreviewUrl} alt="icon" width="200" /> </div>)');
}
return (
<div>
<label>Profile picture: </label>
<input className="Column" type="file" accept="image/*"
onChange={(e) => {fileChangedHandler(e)}} />
// I want to show the selected image here as preview before upload
{ imagePreview }
</div>
)
Hopefully, I will give a clear understanding. What is my intention?

Answer 1

The reason you're having issues is because you're trying to access the state of imagePreviewUrl right after you update it with setImagePreviewUrl - this will not work.
const fileChangeHandler = (event) => {
// ...
reader.onloadend = () => {
// this is asynchronous
// so this definitely wouldn't get
// executed before you get to the `if` statement below
// also, `setImagePreviewUrl` is asynchronous as well
setImagePreviewUrl(reader.result)
}
// ...
if (imagePreviewUrl) {
// the old value of imagePreviewUrl is being used
// here, and since the initial value evaluates to
// `false`, the `setImagePreview` call below
// never gets executed
}
}
Your new value for imagePreviewUrl won't be available immediately because it's asynchronous, so you should probably create a separate hook that listens for changes to imagePreviewUrl and updates imagePreview.
function App() {
const [imagePreviewUrl, setImagePreviewUrl] = React.useState("");
const [imagePreview, setImagePreview] = React.useState(null);
const fileChangedHandler = () => {
setImagePreviewUrl(
"https://interactive-examples.mdn.mozilla.net/media/examples/grapefruit-slice-332-332.jpg"
);
};
React.useEffect(() => {
if (imagePreviewUrl !== "") {
const image = (
<div>
<img alt="fruit" src={imagePreviewUrl} />
</div>
);
setImagePreview(image);
}
}, [imagePreviewUrl]);
return (
<div className="App">
<button type="button" onClick={fileChangedHandler}>
Trigger file change handler.
</button>
{imagePreview}
</div>
);
}
Here's a working example:
CodeSandbox

Answer 2

This is not a recommended approach, what you want is a conditional rendering:
const [imagePreview, setImagePreview] = useState(false);
if (imagePreviewUrl) {
setImagePreview(true);
}
return (
imagePreview && (
<div>
<br /> <img src={imagePreviewUrl} alt="icon" width="200" />
</div>
)
);