Question

I’m working with this JS plugin, and I’ve encountered some syntax I’ve never seen before. I understand what it’s doing, but I’m not sure why it works.

Here’s an example of one instance of it:

settings.maxId != null && (params.max_id = settings.maxId);

Is this just taking advantage of conditionals and the single = ? Is this common syntax for JS?

Answer 1

In JavaScript the = operator is an expression and evaluates the assigned value. Because it is an expression it can be used anywhere an expression is allowed even though it causes a side-effect.
Thus:

settings.maxId != null && (params.max_id = settings.maxId)

Means: If settings.maxId is not null then (and only then, since && is short circuiting) evaluate the right-expression (params.max_id = settings.maxId) which in turn causes the value of settings.maxId to be assigned to params.max_id.
This is much more clearly written as:

if (settings.maxId != null) {
  params.max_id = settings.maxId
}

Happy coding.