Callback function example

Question

I am having a hard time understanding how the callback() function is used in the following code block.

How are we using callback() as a function, in the function body, when function callback() has not been defined?

What are the repercussions of passing true / false as parameters into the callback function below?

I appreciate any clarification, thanks in advance!

socket.on('new user', function(data, callback){     if (nicknames.indexOf(data) != -1){         callback(false);     } else{         callback(true);         socket.nickname = data;         nicknames.push(socket.nickname);         updateUserList();     } }); 

Answer 1

When you pass a function as an argument, it is known as a callback function, and when you return a value through this callback function, the value is a parameter of the passed function.

function myFunction(val, callback){     if(val == 1){         callback(true);     }else{         callback(false);     } }  myFunction(0,  //the true or false are passed from callback()  //is getting here as bool // the anonymous function below defines the functionality of the callback function (bool){     if(bool){         alert("do stuff for when value is true");     }else {         //this condition is satisfied as 0 passed         alert("do stuff for when value is false");     } }); 

Basically, callbacks() are used for asynchronous concepts. It is invoked on a particular event.

myFunction is also callback function. For example, it occurs on a click event.

document.body.addEventListener('click', myFunction); 

It means, first assign the action to other function, and don't think about this. The action will be performed when the condition is met.