How to make images resize as per client device size - reactjs

Question

I have a react app that has many image references ( tags <img src=... /> and css background:url(...)) type.
These images are hosted on Azure Storage.
To speed up my App loading time on various devices (desktops and mobile), I need to resize these images before they hit the client, ie, on the server somewhere.
So far, I can think of the following options:
Pick each image, and produce multiple versions of them for various standard device sizes. Then, pick up each <img src=... /> tag, and, using JS alter the image name, such that the right size of image gets served. This will not work with css.
Use Azure CDN to automatically resize images. I was hoping that resizing would happen automatically, as the CDN portal retrieves the user-agent from the device. Does anyone know if this is true?
Serve images through an Azure function, resizing them on the fly (as suggested here)
Can someone suggest other options they can think of, or a pros / con of the above.

Answer 1

Since you're using javascript, use the window tag. For browsers, the window tab measures the resolution of the browser and you can set the height and width of your image to window.innerHeight and window.innerWidth. There are multiple other ways to do this but this is the easiest and most optimised if your coding project needs to be efficient with the least lines of code necessary.
More info about the window object here : https://www.w3schools.com/js/js_window.asp
P.S. this is only a solution for desktop, for mobile you can use screen.width, screen.height. This might not work on desktop but on a macOS Big Sur device it works, I tried it (This might be because macOS Big Sur is like a mobile optimised interface given that you can even run iOS apps on it but we don't know unless we try). That might be a better option as it is most likely common across all your devices.
More info about the screen object here : https://www.tutorialrepublic.com/faq/how-to-detect-screen-resolution-with-javascript.php
On the off-chance that none of them are common across all of your target devices, try making a detector program with which you can detect the device type and store that in a variable. Then create 2 if statements saying
if(deviceType = iOS){
<img src=..., screen.width, screen.height/>
}else if(deviceType = Windows){
<img src=..., window.innerWidth, window.innerHeight/>
}
Obviously this code won't work but it's just there to show you the flow where you can sort of understand what I meant. You need to integrate it your own way but this was just a way to make it easier as many times people mention that my answers are not easy to understand, just as a safety measure.
The best part of these options is that instead of remade copies of the image itself, this will resize the one, which saves storage space and eliminates the chance of the user using an unexpected display output like a 49" Samsung Odyssey G9 monitor where the resolution is extremely far from what you might have expected and resized. This also means you don't have to create a separate file just to make image resizing code, just the one to detect the OS (not necessary if the screen object works) which would've already been done since this is Azure we're talking about and they always detect their user base.
If you have any queries, please reply back.
Good luck!