Example of JavaScript callbacks.

callbacks.js

// executes a single function
function single(callback){
  callback();
}

function b(){
  console.log("I'm b");
}

function d(){
  console.log("I'm d");
}

//prints: I'm b
single(b);
//prints: I'm d
single(d);

// pass multiple functions

function multiple(fns){
  if (fns.constructor === Array){
    var length = fns.length;
    for (i = 0; i < length; i++){
      fns[i](); //executes ith function
    }
  } else{
    fns(); //executes the function passed to it
  }
}

// prints I'm b
multiple(b);

// prints I'm b followed by I'm d
multiple([b,d]);