There are many choices for backend languages out there, and imho it really depends on the project as a whole. Are you the only developer? Do you just want to hack your home, or are you developing for a customer or even a company? What do you plan to do with it and what languages do you (and the rest of the team) know?
If you are alone, just want to hack, and probably already know JS anyway, NodeJS is one of the quickest ways to get something up and running. You'll need a server, however there are also many freemium hosting services ( Zeit , IBM Cloud , ...).
I haven't played around a lot with PHP lately, so I cannot give a recommendation in that direction. The advantage of PHP is that it can basically run anywhere, even on your regular webspace.
Spring/Java is the way to go for clean, stable applications for customers and especially corporate use. However, it requires you to either have a server or dockerize it for the cloud.
If you feel adventurous, I recommend tinkering a bit with the Rust language, which is the most loved language (for three years in a row) on SO. There are several great frameworks, and I read a lot of good about Actix. You'll need to dockerize the server, though I know that at least Zeit also offers free cloud-hosting for rust servers specifically. Read j's reply below, he already worked with several Rust web frameworks and has real hands-on experience, and check out j's and my conversation in the replies below this post
Python might also be a great idea. It is used for many things, especially data science. It basically works similar to NodeJS. I don't use Python myself, so I think you'll have to dockerize, but there might be cloud-providers for it, too.
If you are a Microsoft-person, of course there also is .NET! You can create a web app in C#, too, and there are many companies which prefer this route, because of the support contracts and compatibility with their environment.
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