If you use any strongly typed language with a halfway decent framework you _don't_ have to think about this because the request will automatically be validated and rejected for invalid input before your code is ever hit.

That’s also true of a decent JS library. The problem here being that the library wasn’t decent.

A library in a strongly typed language would still need to use parameterised queries or escape the strings just like a JS library. And it would be just as easy or difficult to have an incorrect sanitising function.

In fact, the specified string is not invalid input, it would be perfectly valid to store backticks in a string field in MySQL. They just need to be correctly escaped before being submitted to the database.

That's not really true. Pretty much every DB driver I know, in every language, will take prepared statement values that are any valid DB datatype: strings, ints, decimals, etc., and, importantly, JSON values because most DBs now support JSON. Many JS DB frameworks will take any JS object and convert it to a JSON string because that's obviously a trivial operation (it is "JavaScript Object Notation" after all).

Again, the only bug here is the completely incorrect serialization that mysqljs does.

You're missing my point because you're focused on the database query. I'm talking about the web application framework. If this was written in Asp.Net, for example, the password field would be declared as a string and the data binding code within Asp.Net would validate that the field was in fact a string as part of instantiating a model for the strongly typed request body. If someone passed in an object for that field your code for the endpoint, including all of the database interaction, would never be called because the framework would automatically return a 400 response when data binding failed.

I don't think it would actually be possible to use proper prepared statements for a "feature" like this. The whole point is that query is built dynamically from any JSON object. So you could, strictly speaking, build a prepared statement dynamically but you would have the exact same bug.