Understanding callback on this MDN example

the callback is passed by the async package.

Explanation:

As async parallel function takes array or object (in your example) of asynchronous tasks and these async tasks require a callback which will get called when its execution completes or if there is an error. So parallel function provides these callback functions and will call your callback ( provided as a second parameter to parallel function call) when all of them are called or got an error in any of them.

You can check the detailed explanation from here - https://caolan.github.io/async/v3/docs.html#parallel

Update:

Consider parallel as a wrapper function like:

function parallel(tasks, userCallback) {    let tasksDone = [];    function callback(err, data){ // this is the callback function you're asking for        if(err){            userCallback(err); // calling callback in case of error        }else {            tasksDone.push(data);            if(tasks.length === tasksDone.length){                userCallback(null, tasksDone); // calling callback when all the tasks finished            }        }    }    tasks.forEach(task => {        task(callback); // calling each task without waiting for previous one to finish     })}

Note: This is not a proper implementation of parallel function of async, this is just an example to understand how we can use callback function internally and what is its usecase