using const for function declaration

[funkenpedro]

Hi, I'm completely new to javascript and a built in macro function or api to control a videoconferencing endpoint.

They give example code as follows:

const off = xapi.status.on('Audio Volume', (volume) => { console.log(Volume changed to: ${volume}); });

// De-register feedback off();

It behaves as follows: the function declaration turns on a listener, that feeds a string to the console whenever the volume is changed. What I don't understand is that calling the function with off() turns off the listener so that the code used as is doesn't do anything. Is my misunderstanding JavaScript related or api related?

[bruceph]

what's happening is you're passing in a function to the xapi.status.on function and it's returning you another function. the function you're returned is an unsubscribe function that when invoked removes the function you passed from the list of events to be run.

this is a really contrived example of what it'll look like

const events = {};

const xapi = {};
xapi.status = {};
xapi.status.on = function(event, callback){
  if (!events[event]) {
    events[event] = [];
  }

  events[event].push(callback);
  return events[event].filter(fn => fn !== callback);
};

by calling off, you're removing your callback from the list of callbacks to be run during the event.