Signals are similar to calling methods, you can call them.

Javascript -> GDScript

In GDScript we have this signal defined:

signal random_signal(some_vec: Vector2i)

We can call it from Javascript:

$g.example_obj.random_signal([4,5])

Connection

You can connect to GDScript signals in javascript and provide a callback:

$g.example_obj.random_signal.connect((res) => {
  console.log("signal from godot: ", res);
});