how to understand callbacks and async.parallel in node

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I'm very new to JavaScript and callbacks, so my apologies if this is a stupid question. Based on the async docs for parallel, I came up with this example code that executed the expected way based on the docs:

async = require('async')

async.parallel([
    function(callback){
        setTimeout(function(){
          callback(null, 'one');
        }, 800);
    },
    function(callback){
      setTimeout(function(){
        callback(null, 'two');
      }, 100);
    }
],
    function(err, results){
      console.log(results)
    })       

Running this with node foo.js prints a results array ['one', 'two'] as the docs indicate it will. The thing I don't understand is how exactly this works: when you pass callback as a parameter to each function and then callback is called as callback(null, res), what exactly is being called her? I've not actually defined any function callback, nor have I passed any sort of operating callback function as a parameter, yet everything magically works fine. Am I totally missing the point here? Or is this actually the under-the-hood magic of async?

asked 42 secs ago

user139014

338