以下内容主要摘自［1，2］
（1）In javascript, functions are first-class objects, which means functions can be used in a first-class manner like objects, since they are in fact objects themselves: They can be “stored in variables, passed as arguments to functions, created within functions, and returned from functions”。
(2)Callback functions are derived from a programming paradigm called functional programming. At a simple and fundamental level, functional programming is the use of functions as arguments. Functional programming was—and still is, though to a much lesser extent today—seen as an esoteric technique of specially trained, master programmers.
(3)When we pass a callback function as an argument to another function, we are only passing the function definition.
(4)If callback function is a asynchronous function[自己定义的callback函数如果调用了异步函数库，则该函数是一个异步函数；否则，同步函数.例如：node中读取文件的两个函数 fs.readfile() vs fs.readfileSync()], then callback function will be executed later than those code behind the function which called the callback function.
(5)every function in JavaScript has two methods: Call and Apply.
Callback functions are extremely important in Javascript. They’re pretty much everywhere. Originally coming from a more traditional C/Java background I had trouble with this (and the whole idea of asynchronous programming), but I’m starting to get the hang of it. Strangely, I haven’t found any good introductions to callback functions online — I mainly found bits of documentation on the call() and apply() functions, or brief code snippits demonstrating their use — so, after learning the hard way I decided to try to write a simple introduction to callbacks myself.
Functions are objects
To understand callback functions you first have to understand regular functions. This might seen like a “duh” thing to say, but functions in Javascript are a bit odd.
Functions in Javascript are actually objects. Specifically, they’re Function objects created with the Function constructor. A Function object contains a string which contains the Javascript code of the function. If you’re coming from a language like C or Java that might seem strange (how can code be a string?!) but it’s actually run-of-the-mill for Javascript. The distinction between code and data is sometimes blurred.
1 // you can create a function by passing the
2 // Function constructor a string of code
3 var func_multiply = new Function("arg1", "arg2", "return arg1 * arg2;");
4 func_multiply(5,10); // => 50
One benefit of this function-as-object concept is that you can pass code to another function in the same way you would pass a regular variable or object (because the code is literally just an object).
Passing a function as a callback
Passing a function as an argument is easy.
01 // define our function with the callback argument
02 function some_function(arg1, arg2, callback) {
03 // this generates a random number between
04 // arg1 and arg2
05 var my_number = Math.ceil(Math.random() *
06 (arg1 - arg2) + arg2);
07 // then we‘re done, so we‘ll call the callback and
08 // pass our result
09 callback(my_number);
10 }
11 // call the function
12 some_function(5, 15, function(num) {
13 // this anonymous function will run when the
14 // callback is called
15 console.log("callback called! " + num);
16 });
It might seem silly to go through all that trouble when the value could just be returned normally, but there are situations where that’s impractical and callbacks are necessary.
Reference
[1]http://recurial.com/programming/understanding-callback-functions-in-javascript/