Understanding callback function in JavaScript

The following are mainly excerpted from [the]

(1) in JavaScript, functions is first-class objects, which means functions can be used in a first-class manner like object s, since they is in fact objects Themselves:they can is "stored in variables, passed as arguments to functions, created Within functions, and returned from functions ".

(2) Callback functions is derived from a programming paradigm called functional programming. At a simple and fundamental level, functional programming are the use of the functions as arguments. Functional programming Was-and still are, though to a much lesser extent today-seen as an esoteric technique of specially t Rained, master programmers.

(3) When we pass a callback function as an argument to another function, we is only passing the function definition.

(4) If callback function is a asynchronous function[its own defined callback function if an asynchronous function library is called, the function is an asynchronous function; For example: two functions for reading a file in node Fs.readfile () vs Fs.readfilesync ()], then callback function would be executed later than those code behind the function whi Ch called the callback function.

(5) Every function in JavaScript has a methods: Call and Apply .

Callback functions is extremely important in Javascript. They ' re pretty much everywhere. Originally coming from a to traditional C/java background I had trouble with this (and the whole idea of asynchronous PR ogramming), 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 T He call () and apply () functions, or brief code snippits demonstrating their use-so, after learning the hard I decide D-to-try-to-write a simple introduction to callbacks myself.

Functions is objects

To understand callback functions your first has to understand regular functions. This might seen like a "duh" thing to say, but functions in Javascript is a bit odd.

Functions in Javascript is 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, might seem strange (how can code is a string?!) but it ' s actually run -of-the-mill for Javascript. The distinction between code and data is sometimes blurred.

1    //  can create a function by passingthe 2    /  function constructor a string of Code3    varnew Function ("Arg1", "arg2", "return arg1 * ARG2;" ); 4    //  =

One benefit of this function-as-object concept is so you can pass code to another function in the same-a-do you would pas s 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 are easy.

01//define our function with the callback argument02functionsome_function (arg1, Arg2, callback) {03//This generates a random number between04//Arg1 and Arg205varMy_number = Math.ceil (Math.random () *06 (ARG1-ARG2) +arg2);07//Then we ' re do, so we'll call the callback and08//Pass Our result09callback (My_number);10    }11//Call the functionSome_function (5, 15,function(num) {13//This anonymous function would run when the14//callback is calledConsole.log ("Callback called!" +num);16});

It might seem silly to go through all this trouble when the value could just is returned normally, but there is situation s where that ' s impractical and callbacks is necessary.

Reference

[1]http://recurial.com/programming/understanding-callback-functions-in-javascript/

[2]http://javascriptissexy.com/understand-javascript-callback-functions-and-use-them/? wpacfallback=1&wpacrandom=1413199645166

Understanding callback function in JavaScript