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functions returned from other functions in javascript

I am new learner of Javascript and I got stuck in "functions returned from other functions".
The code is as below.
var createScream = function(logger) { return function(message) {
logger(message.toUpperCase() + "!!!")
}
}
const scream = createScream(message => console.log(message))
scream('functions can be returned from other functions')
scream('createScream returns a function')
scream('scream invokes that returned function')
Console output:
"FUNCTIONS CAN BE RETURNED FROM OTHER FUNCTIONS!!!" "CREATESCREAM
RETURNS A FUNCTION!!!" "SCREAM INVOKES THAT RETURNED FUNCTION!!!"
Question: why it will work this way? what is the process of executing?
Does function scream have any parameter? how does it work?
This may be very easy, I searched but no clear explanation.
Can someone please give me a hint or explain the whole execution process in details?
Thanks in advance!

First, Higher order functions are not about functional languages. Functions that operate on other functions i.e. taking a function as an argument and returning another function, are called higher order functions.
Your snippet:
You created an anonymous function with argument logger, which returns another anonymous function and bind that to variable createScream.
Then you called createScream with a function as an argument. storing the returned value in const scream. At this time you are returned a function with argument logger being the fat arrow function you passed.
Now you called scream with strings. Remember scream is also a function because in the earlier step the function call returned another function. That function has access to logger argument passed to the outer function. This is an example of closure.
When you call scream with strings. The returned function is called which is logging the message in uppercase plus !!!.
It will take sometime to get what is happening here, If you have limited programming experience. Read about higher order functions, lexical scope, closure.

In JavaScript and other functional languages, you will hear the term that functions are "first-class citizens" meaning that functions can be passed around as values or referenced by other functions.
It's normal for this to take a bit of time to grok, but try to walk through your code step by step and you'll get it:
The createScream function accepts a function as input (logger)
The createScream function then returns another function that accepts its own input (message)
The returned function uses the original input function (logger) as well as the second input (message)
And consider the execution order in usage as well:
scream invokes createScream with a console.log function
now when you invoke scream with a message, that message will first be changed as per the definition in createScream
then once the message is changed, it is invoked by the logger function console.log

Difference between passing a callback versus a function

I'm curious, is there any difference between the two functions below?
Is one just passing a function inside an anonymous function when ready whereas the other is passing an actual named function when ready?
Example:
<p>Not loaded yet.</p>
First method:
function newName() {
$( "p" ).text("The DOM is now loaded and can be manipulated.")
}
$(document).ready(function() {
newName()
});
Second method:
function newName() {
$( "p" ).text( "The DOM is now loaded and can be manipulated.")
}
$(document).ready(newName());
Is one more correct than the other?

Is one just passing a function inside an anonymous function when ready whereas the other is passing an actual named function when ready?
Yes and no.
I'm curious, is there any difference between the two?
The first is passing an callback function that is executed when the event fires. When that even fires is explained here.
The latter passes the resolved return value (which is undefined because newName has no return statement), which isn't the same as the second. The reason it passes the return value to ready is because you invoke the function immediately (which may seem like it works), which then passes the return value. To make them functionally equivalent, do:
$(document).ready(newName);
This will pass a function reference, and won't invoke it. Also, as mentioned before, invoking as ready(func(); func2();) is not valid syntax.

Effectively there’s no difference between
$(document).ready(function(){
newName();
});
and
$(document).ready(newName);
There’s a small technical difference: in the first snippet, an anonymous function calling newName is passed, in the second snippet, the named function newName itself is passed.
Functions are just callable objects that can be referenced by their name or the name of a variable holding the function.
Note that $(document).ready(newName()); is calling newName — immediately — and passing the return value to ready. This only makes sense if newName returns another function, which in this case it doesn’t.
The title also mentions something like $(document).ready(newName){}. I’m assuming that’s a typo, since that’s not valid syntax.
$(document).ready(newName(); otherName(); otherName1()); also isn’t valid syntax.

JavaScript examples, clarify the langue pattern design for functions with-in functions

I am learning JavaScript and becoming confused by the logic of the code examples. From codecademy. Why are there function set-ups in function calls?
I'm quite confused. I am moving from a simplified C-like langue.
The JavaScript example
var main = function(){
$('.article').click(function(){
$('.description').hide();
$(this).children('.description').show();
});
};
My understanding:
- main is a function name with a return type of var.
$('.article') is a element/object/or class object.
.click() is a call to a member function
But:
???:
.click(function(){
$('.description').hide();
$(this).children('.description').show();
});
This seems to be a newly on the spot created function to run When/If click() is activated or run.
The way I used to think is like this:
var *p_obj = $('.article');
var *p_obj = $('.description');
var do_click()
{
p_obj2.hide();
p_obj.children(p_obj2).show();
}
var main(){
p_obj.click(do_click);
}
Function main() looks at p_obj and calls click().
Click() evaluates to true/false and run the pointer_to function do_click().
Function do_click() looks at the p_obj2 and calls hide(), which performs an action of hiding the p_obj2.
Function do_click() also looks at p_obj and uses children to scope focus to p_obj2, then it runs show(), which preforms an action of displaying p_obj2.
I do realize my C-like example is wrong and odd. I realize my terminology is wrong or otherwise used incorrectly.
The way this design looks seems like I must write extended functionality on-the-spot for every call to .click(), so if-then .click() is run on 3 different items, I'm creating different extended functionality for each object. But I would normally create a single function that varies it's internal execution based on the object or condition click() calls it by.
This set-up seems alright if the code a relatively simple or short, but on-the-spot functional seems like overworking for longer code and code where the functionality repeats but the objects change.
Am I thinking about JavaScript functions with-in functions correctly and is this a design goal of the langue to add long repeating extended functions with-in functions?

Here, you should understand 2 things:
passing functions as arguments
anonymous functions
The first concept is particulary important because callbacks are popular in JavaScript, so let me explain it for callbacks. Imagine we have 2 functions getStuffFromWeb and processStuff. You probably expect that they are used like this:
var result = getStuffFromWeb();
processStuff(result);
But the issue here is waiting for getStuffFromWeb may take some time (the server is busy), so instead they are usually used in a "when you finish, call this function" manner, which is:
var getStuffFromWeb = function(params,callback) {
...
callback(result);
};
getStuffFromWeb(someParams,processStuff);
Well, in fact the structure of getStuffFromWeb will be different, most likely something like this:
var getStuffFromWeb = function(params,callback) {
requestObject.make_request(params)
.onSuccess(callback);
};
So when getStuffFromWeb is called, it starts to listen to response while the code after getStuffFromWeb(someParams,processStuff); goes on evaluating. When the response comes, it calls the callback function to process the data further using the procedure we have defined (processStuff).
The second concept is rather simple: you may of'course write smth like
var processStuff = function() {...};
var getStuffFromWeb = function(params,callback) {
requestObject.make_request(params)
.onSuccess(callback);
};
getStuffFromWeb(someParams,processStuff);
but if you use processStuff only once, why define a named function? Instead, you can just put the very same expression inside the onSuccess param like this:
var getStuffFromWeb = function(params) {
requestObject.make_request(params)
.onSuccess(function() {...});
};
getStuffFromWeb(someParams);
This looks exactly like if we took the value of processStuff and put it directly to the onSuccess's argument (and that's called anonymous function). And also we got rid of an extra argument of getStuffFromWeb.
So basically that's it.

Simple answer is that the second argument of click() requires a callback function.
This can be a named function passed as reference as in your p_obj.click(do_click); example or it can be an anonymous function with self contained logic. Anonymous functions are very common in javascript
It's the same thing just with 2 different ways of declaring the callback.
Note that the only time you would return anything from an event handler function would be to return false which effectively prevents the default browser event (url opening from href or form submit for examples) and stops event propagating up the DOM tree

main is a function name with a return type of var.
No. main is a variable which is assigned an anonymous function. The function name would go between the keyword function and the () containing the argument list.
It has no return statement so it returns undefined.
$('.article') is a element/object/or class object.
It is a call to the function $ with one argument. The return value is a jQuery object.
.click() is a call to a member function
Pretty much. In JavaScript we call any function that is the value of a property of an object as method.
This seems to be a newly on the spot created function
function () { } is a function expression. It creates a function, exactly like the one used to assign a value to main earlier. This question is worth reading for more on the subject.
When/If click() is activated or run.
The click function is called immediately. The new function is passed as an argument.
The purpose of the click function is to bind a click event handler so that when a click event hits the element later on, it will trigger the function passed as an argument.
I do realize my c -like example is wrong and odd. I realize my terminology is wrong or otherwise used incorrectly.
Leaving aside vagaries of syntax. The main difference here is that the click event handler function is that the event handler function is stored in an intermediary variable.
You can do that in JavaScript just as easily, and then reuse the function elsewhere in the code.
var main = function(){
function show_specific_description() {
$('.description').hide();
$(this).children('.description').show();
}
$('.article').click(show_specific_description);
show_specific_description.call($(".article").last()[0]);
};
main();
is this a design goal of the langue to add long repeating extended functions with-in functions?
No. Passing a function expression as an argument is a convenient way to be more concise when you don't want to reuse the function. It's not the only way to pass functions about.

main is currently a function.
It is possible to be overwritten (even to a different type). var is not the return type, it's a statement that main is a variable.
All values should be declared as variables, within the highest scope you intend them to be used (in JS, scope typically means functions, not blocks).
You have the right idea, suspecting that the function gets passed in, and called at a later point in time (and this is actually one of the harder parts for people to get, coming from certain other languages). You'll see this behaviour all through JS.
One key thing to keep in mind in this language (you haven't hit it yet, but you will) is that JS is lexically scoped.
function getInnerX () {
var x = 5;
function getX () {
return x;
};
return getX;
}
var x = 10;
var getX = getInnerX();
console.log(getX()); // 5
The function getX inside of getInnerX has access to the references around it, at the point where it's defined (not where it's called), and thus has live access to the inner x, even if its value changes over time.
This will be another important piece of understanding what you see going on in the language, especially in the case of callbacks.

Do I have an accurate understanding of callback functions?

I've been a Javascript tinkerer for quite some time and recently I have been trying to have a real good understanding of the code I write. I want to get really good at writing Javascript. One of the things that's confused me a little bit was the usage of callback functions. I've just avoided using callbacks up until now. Right now, I'd like to know if my understanding of how a basic callback works is accurate and also if my understanding of parameters and arguments is accurate and finally, what the purpose is of using the callback in this scenario. Is there a benefit to using the callback function I've created below? If not, when are callback functions most beneficial? I realize what I've written below is quite lengthy so for the TLDR people: Am I right in thinking It's as if I created the function "theCallback" within "myFunction"? If so, what is the benefit in the scenario below? Why not just call a function? why must the function being called be an argument in the "myFunction" function?
If the scenario below doesn't make it necessary for using a callback, when is it necessary and what are the benefits? Thanks.
Here's how I see the code below: I've created a factory function "myFunction" with two parameters "x" and "callback" and parameters are pretty much variables that will be stored with arguments when the function is called. I also created another factory function "theCallback" with two parameters "x" and "theValue". Now since these are factory functions, regardless of them being underneath a function call to "myFunction", they will be hoisted above the function call so there are no problems. Had I created a function expression with a variable, they would not be hoisted. So now my call to "myFunction" has two arguments that the parameters in it will be defined as, "3" and "theCallback". The "theCallback" argument is a function. This function has the two parameters "x" and "theValue". When "myFunction" is called, "x" is stored with the argument "3" and "callback" is stored with the function "theCallback" and then the code within "myFunction" is executed which is simply a variable declared and assigned a value of a string that contains the text "for the callback" and the callback variable (I'm viewing as a variable), which was a parameter that is now an argument which is "theCallback" function I created which also contains parameters itself, is called, and given arguments for it's code to execute. In this case, the code it will execute calls the built in alert function (or method of the window object?) and gives the alert function a string argument, followed by the
alert function called again with the argument of "x" which is stored with "3" and then again alert is called and given the argument of "theValue" which contains the string "for the callback". Now when "theCallback" function is passed as a second argument to "myFunction" and it's stored in "callback", It's as if I created the function "theCallback" within "myFunction" right? If so, what is the benefit in the scenario below? Why not just call a function, why must the function being called be an argument in the "myFunction" function?
If the scenario below doesn't make it necessary for using a callback, when is it necessary and what are the benefits?
myFunction(3, theCallback);
function myFunction(x, callback){
var value = 'for the callback';
callback(x, value);
}
function theCallback(x, theValue){
alert('this is the callback function');
alert(x);
alert(theValue);
}

Is there a benefit to using the callback function I've created below?
That's a little subjective. Using a callback is unnecessary there. Normally you would return data from one function and pass it to the other.
when are callback functions most beneficial?
When you need to deal with the data after an event has fired, instead of immediately. e.g. when a button is clicked or an HTTP response has fired.
myButton.addEventListener("click", myCallbackFunction);
I've created a factory function "myFunction"
No. That is just a function. Factory functions create class objects.
they will be hoisted
Function declarations (as opposed to function expressions) are hoisted. This isn't relevant to callbacks though. A callback is basically a function passed as an argument, it doesn't matter how it was created.
what are the benefits
The benefits are that you can have different things happen when the event fires depending on the circumstance.
Take the click event example above. Often you'll want clicking on different things to have different effects, not the one thing that addEventListener was hardcoded to make happen by the browser vendor.

Meaning of statement in JS

Trying to figure out what this means in javascript.
(function(document) { ... } )(document);
It is isn't using jQuery, so is this just a javascript way of making this wait till document is ready to execute?
Thanks.

This won't wait for the document to be ready, this will execute the content of the function immediately. Putting the function definition in parenthesis makes it an expression, which returns a value being the function, making it directly executable. This pattern is called an Immediately-Invoked Function Expression (IIFE).
This is probably used in conjunction with a minifier like the Closure Compiler.
Inside the function, document is a local variable. This makes it possible for the minifier to reduce its name to a one or two character name.
Note also that all variables defined inside the function will be local : they won't leak in the global scope, which may be interesting if this is only part of the script.

This creates an anonymous function that takes a single argument, and immediately calls it passing document as the argument.

This:
function(document) { ... }
creates a function taking one paremeter.
This:
(function(document) { ... })
makes it (the code, not the function) a valid expression. See here.
This:
(function(document) { ... } )(document);
calls that function with document as a parameter.
It's a basic modularization pattern. In different environments you could've passed some other object instead of document, but nothing inside that function has to know bout it.

This is called self-executed function. It evaluates the anonymous function taking a parameter called document with that parameter passed in.