How do I port this function definition to a C# declaration?

this is one of the more painful aspects of JavaScript. What it refers to will depend on how and where the function is called.

If you just call FollowEl(x, s), then this will be the global object. In a browser, that would be the window object. NB: This happens regardless of whatever this refers to in the calling scope.

FollowEl(x, s); // this === window

If you call FollowEl.call(foo, x, s), or FollowEl.apply(foo, [x, s]), then this will refer to the first argument passed in.

var foo = { id: 42 };
FollowEl.call(foo, x, s); // this === foo
FollowEl.apply(foo, [x, s]); // this === foo

If you bind the function to an object, you'll get a new function returned. When you call that new function, it will call the original function, but with the bound object as this.

var foo = { id: 42 };
var boundFn = FollowEl.bind(foo);
boundFn(x, s); // this === foo

If you write it as an arrow function, then it should inherit this from the calling scope.

const FollowEl = (N, sets) => {
    this.N = N;
    this.sets = sets;
};

var foo = {
    id: 42,
    fn: function(N, sets){ 
        FollowEl(N, sets); 
    }
};

foo.fn(); // this === foo

However, this doesn't seem to work for me.

this - JavaScript | MDN[^]
Function.prototype.bind() - JavaScript | MDN[^]
Arrow function expressions - JavaScript | MDN[^]
Scope in JavaScript - Digital Web[^]


In short, you'll need a lot more context before you can convert this to C#. :)