node.js聊天应用程序中callback（）的含义或源代码

Question

I'm trying to go through the source code for the node chat demo seen here . In the server.js file and fu.js file there is a function referenced as callback() seen here:

function loadResponseData(callback) { 
if (body && headers && !DEBUG) { //if they already have value
  callback();
  return;
}...

but as far as I can tell this function is never defined and I cannot find it as a module function of node.js, a function of jquery or a standard function for javascript.

I think I understand how callback functions work but I'm not familiar with this call and it is used frequently enough in this app that I would like a firm understanding of what it is and where it comes from.

So my question is three fold: 1) where is the function based: (javascirpt, jquery, node.js, particular to this app) 2) where can I find the source code for this function? 3) how is this function interacting with the functions it is called in?

Answer 1

It is the argument of the loadResponseData function. If you call loadResponseData like this:

loadResponseData(function () {
      res.writeHead(200, headers);
      res.end(req.method === "HEAD" ? "" : body);
    });

then callback() in loadResponseData will execute

res.writeHead(200, headers);
res.end(req.method === "HEAD" ? "" : body);

EDIT to clarify the question in comments:

You could say it's a feature of JavaScript. The important thing here is that JavaScript is a functional language: it has functions as a data type in their own right. Thus, you can save them in variables (and indeed, that's all every function name in JS is - a variable with a function as its content), and pass them along in an argument list (as is demonstrated here). There is nothing magical about the name callback - it could have as well been fn or whoopsie7 . To demonstrate:

var doubleAndOne = function(a) {
  return a * 2 + 1;
}
function doItTwice(k, whoopsie7) {
  whoopsie7(whoopsie7(k));
};
doItTwice(5, doubleAndOne); // result is 23

function(...) {...} is called an anonymous function: it is pure function value, taking some arguments and doing something with them, but it is not assigned to any name. To make a function with a name, you can assign this to a variable, or pass it as a parameter, just like you could with any other value. For example, there is very little difference between:

var five = function() { return 5; };
var doubleFuncValue = function(fn) { return fn() * 2; };
doubleFuncValue(five); // result 10

and

var five = 5;
var doubleNumValue = function(n) { return n * 2; };
doubleNumValue(five); // result 10