Develop Reference

JavaScript: Callback Accessing another parameter

In this example it seems that a callback is accessing another parameter (without having to provide the argument again).
Excerpt from the link above
var SimplePropertyRetriever = {
getPrototypeEnumerables: function(obj) {
return this._getPropertyNames(obj, false, true, this._enumerable);
},
_enumerable: function(obj, prop) {
return obj.propertyIsEnumerable(prop);
},
_getPropertyNames: function getAllPropertyNames(obj, iterateSelfBool, iteratePrototypeBool, includePropCb) {
...
}
}
As seen:
this._enumerable is passed to _getPropertyNames
_enumerable accepts a parameter called obj
obj is not explicitly passed down though, so it seems that when the callback is passed to _getPropertyNames it somehow accesses its first argument, which is obj
To test it, I tried the below, which did not work.
function myFunc2(para, callback) {
console.log(`Para: ${para}`);
callback();
}
myFunc2(42, (para) => console.log(para));
Any idea what I am missing here?

I would suggest you understand Closures and Variable Scopes in JavaScript.
For the code snippet you mentioned above, since your callback needs an arg 'para' you can pass it and log it. If you don't want to pass it you can just add it to global object( 'window' object in case of browser).
This is because your anonymous callback function expects 'para' variable first in it's own body, next it will try to search in it's parent's body which in this case is global object.
For the below example,
function myFunc2(para, callback) {
window.para = para;
console.log(`Para: ${para}`);
callback();
}
myFunc2(42, () => console.log(para));

How to pass a callback function with multiple possible parameter lists

Here are two callback function:
function callback_a(){
alert('a');
}
function callback_b(p){
alert('b says'+ p)'
}
If I want use callback_a
function test(callback){
if(condition){
callback();
}
}
test(callback_a);
But the function test isn't applicable to callback_b, So how to implement a common function that you can passing some callbacks function with multiple possible parameter lists.

There are three options:
The easiest way is to use spread operator:
function test(callback, ...callback_args) {
callback(...callback_args);
}
in this case the invocation of test for function callback_b would be like this:
test(callback_b,"b")
The second way is using arguments which are scoped to any function in JavaScript:
function test(callback) {
callback.apply(null, arguments.slice(1));
}
the invocation of test for function callback_b would be the same:
test(callback_b,"b")
Another options is to use partially applied functions. In this case you should define b_callback like this (ES6 syntax):
let callback_b = (p) => () => void{
alert('b says'+ p)'
}
or without ES6:
function callback_b(p) {
return function(){
alert('b says'+ p)'
}
}
and invoke it like this:
test(callback_b("b"))

There is a special object called arguments that gets created when a function is invoked. It's an array-like object that represents the arguments passed in to a function:
It can be used like this:
test();
// no arguments passed, but it still gets created:
// arguments.length = 0
// arguments >> []
test(a);
// ONE argument passed:
// arguments.length = 1
// arguments >> [a]
test(a,b,c,d);
// FOUR arguments passed:
// arguments.length = 4
// arguments >> [a,b,c,d]
Knowing this, one can call a callback with the rest of the arguments passed in from the parent function using apply like this:
function test(callback) {
callback.apply(null, Array.prototype.slice.call(arguments, 1));
}
// arguments passed into test are available in the function scope when
// .slice is used here to only pass the portion of the arguments
// array relevant to the callback (i.e. any arguments minus the
// first argument which is the callback itself.)
//
// N.B. The arguments object isn't an array but an array like object so
// .slice isn't available on it directly, hence .call was used here)
Might be worth reading up on:
The arguments object
Function.prototype.apply, Function.prototype.call and Function.prototype.bind as they are way to bind a context and arguments to a function (i.e. they'll work with the arguments object to call a function where you may not know how many arguments will be passed)

So how to implement a common function that you can passing some callbacks function with multiple possible parameter lists.
Basically, you don't. The function receiving the callback is in charge of what the callback receives as arguments. When you call Array#forEach, it's Array#forEach that decides what arguments your callback gets. Similarly, String#replace defines what it will call its callback with.
Your job is to say what test will do, what it will call its callback with. Then it's the job of the person using test to write their callback appropriately. For instance: You might document test as calling the callback with no arguments. If the caller wants to use callback_b, then it's up to them to handle the fact that callback_b expects a parameter. There are several ways they can do that:
The could wrap it in another function:
test(function() {
callback_b("appropriate value here");
});
...or use Function#bind
test(callback_b.bind(null, "appropriate value here"));
...but it's their problem, not yours.
Side note: If they pass you callback_b and you call it without any arguments, you won't get an error. JavaScript allows you to call a function with fewer arguments than it expects, or more. How the function handles that is up to the author of the function.

You can pass an anonymous function as the callback that will itself return your desired callback function with parameters.
test(function() { return callback_b(' how are you'); });
see this working snippet that will first use callback_a, then callback_b (with parameter) as the callback:
function callback_a(){
alert('a');
}
function callback_b(p){
alert('b says'+ p);
}
function test(callback){
if(true){
callback();
}
}
test(callback_a);
test(function() { return callback_b(' how are you'); });

You can pass the parameter while calling the callback
function test(callback){
if(condition){
callback();
}
else if(other condition){
callback("b");
}
}
test(callback_b);
You can write your callback function like
function callback_a_b(){
if(arguments.length){
var arg = [].slice.call(arguments);
alert('b says'+ arg[0])
}
else{
alert('a');
}
}

You can pass array of parameters as second param of test function or in ES6 use spread operator read more here
function test(callback, params){
if(condition){
if (params === undefined){
callback();
} else {
callback.apply(null, params); //params must be array
//ES6: callback(...params);
}
}
}
test(callback_a);
test(callback_b, [" whatever"]);

I've just checked in my browser (ffox 51.0.1) that the following works:
function test(callback,other_args){if(condition){callback(other_args);}}
results:
condition=true
test(callback_a)
=> shows the alert with 'a'
condition=false
test(callback_a)
=> doesn't show anything
condition=true
test(callback_b,"pepe")
=> shows the alert with 'b sayspepe'
condition=false
test(callback_b,"pepe")
=> doesn't show anything

nodejs callback don't understand how callback result come through argument

Seen and run code below, thought I understand closures... How that 'avatarUrl' in callback argument is received in 'avatar'which is function argument too. I know this is common pattern, just can't get it yet
var GitHubApi = require('github');
var github = new GitHubApi({
version: '3.0.0'
});
var getUserAvataWithCallback = function(user, callback) {
github.search.users({q: user}, function(err,res) {
if (err) { callback(err, null);}
else {
var avatarUrl = res.items[0].avatar_url;
callback(null, avatarUrl);
}
});
};
getUserAvataWithCallback('irom77', function(err,avatar) {
console.log('got url with callback pattern', avatar);
})

So, callbacks are an underlying and integral concept in javascript, so it is important that you understand a few concepts. Look at this example:
// This is the function definition for "foo"
//here the callback argument refers to
//the second argument in the function call at
//the bottom which is a function
var foo = function(arg, callback) {
if(arg%2 != 0)
callback("arg is odd", arg)
else
callback(null, arg)
}
//Function call to foo
foo(2, function(err, num) {
if(err)
console.log(err)
else
console.log(num)
}
So, in the example above, you can think of the function call as a call with two parameters, the integer 2 and a function.
In the function definition:
The integer is referred to as "arg" and the function is referred to as "callback".
When callback("arg is odd", arg) is executed, the function is called with:
err = "arg is odd"
num = arg
When callback(null, arg) is executed, the function is called with:
err = null
num = arg
The important thing to remember here is that in javascript, functions can be passed as arguments to other functions. Do some further reading here.

The name of the argument passed to a function does not need to be the name of the argument in the definition of the function, the argument in the definition is the name of the variable that will be initialized inside the scope of given function. The argument declaration will receive the value passed at the second position of the function call (as per the code you've provided) and you will be able to access it inside the scope with that name. You can:
function foo(arg1, arg2) {
console.log(arg1, arg2);
}
foo(true, true); // will output true, true
foo(0, 1); //will output 0, 1
foo('shikaka', 1); //will output "shikaka", 1
var bar = "shikaka";
foo(bar, "shikaka"); //will output "shikaka", "shikaka"

can we give a callback with parameters?

function abc(arg1,arg2, callback){
console.log(arg1 + arg2) // I am doing something really interesting
if(callback){
callback(arg1+ arg2)
}
}
function add(){
console.log("from call back " + arguments[0])
}
var a =10;
abc(1,2, add)
this works fine, but if we need to send some additional arguments to the call back, what should we do??
Here, Apart from (arg1+ arg2) I need some other arguments to be set from the caller of abc to the callback
And, what is the difference between abc(1,2,add) and abc(1,2,add()) ??
Thanks :)

abc(1,2,add) => Giving the function argument as "function type". This is like giving a pointer to the function for later invoking it.
abc(1,2,add()) => Calling the add() function and giving its return value as argument.
Do you need that the callback support more than an argument? Since JavaScript is a dynamic language, just call the same callback function with more arguments:
callback(1,2,3,4)
callback(1,2,3,4,5)
callback(1,2,3,4,5,6).
JavaScript isn't strict with function signatures: functions have as many arguments as the caller gives to them.
For example, if you've a callback function like this:
function(a, b) {
}
And later you call it this way:
function("hello"); // Where's the second argument??
JavaScript won't complain. Simply b will be undefined.

the difference between add and add():
add is a function and add() is the return value of the function.
if you want to get more arguments of the function. use the arguments object
every function has an arguments object
arguments[0] == 1 arguments[1] == 2 arguments[2] == add.

You can call your callback with parameters from your original function and you can also provide the parameters for the callback inside your function call:
function f1 (args, callback, params) {
callback.apply(this, params); // Params needs to be an array
callback.apply(this, [args]); // This would call callback with 1 param - args
callback.call(this, args); // This is the same as the line above
}
function f2 () {
for (item in arguments) {
console.log(item);
}
}
f1('arg', f2, [1, 2, 3]);
If you call your function with a function call inside the parameters then it would immediately evaluate and it would not be executed as a callback.

The difference between abc(1,2,add) and abc(1,2,add()) is that in the second case, it's not abc that calls add. The consequence is that add executes much sooner than expected and without any arguments.
The usual way to pass extra parameters to a callback is to create an anonymous function that closes over the parameters:
var a=10;
var b=20;
abc(a, b, function(text){
add(a, b);
console.log(text); //logs "abc"
});

How can I pass function arguments with a function pointer in Javascript

This is a syntax question I assume. Using function pointers is new to me, and required for a chunk of code I am working with. I want to pass some arguments to LoadResourceList to explain how to handle the data. LoadResourceList is a callback for when the ajaxObject has finished loading.
var getResourceList=new ajaxObject("resources.json",LoadResourceList); // Load Resource List
getResourceList.update();

You can pass the arguments for LoadResourceList into ajaxObject aswell:
function ajaxObject(json, callback) {
// code
if( typeof callback === 'function' ) {
var args = Array.prototype.slice.call(arguments, 2);
callback.apply(this, args);
}
}