Javascript Closure Example

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One of the biggest passwords in JavaScript is shutdown. He is the subject of many questions on job interviews at FAANG companies. In this article, we’ll talk about fence and goal, illustrate the concepts with simple examples, and then conclude with a sample question from an interview with one of the biggest tech giants.

When someone tells you that something is or is not part of a project, what does it mean?

I would like to think of a periscope or a telescope when I think of the answer to this question. These tools show us all kinds of things within the limits of the lens he’s got: he’s in field. If it is out of range, you cannot see past the diameter of the lens. And to illuminate something outside the diameter is not possible. You should think about this as we talk about three very important and distinct types of scopes in JavaScript: local, global, and lexical.

Local scope

The local scope is the smallest of the three scopes we’ll be discussing today. When we declare a function, anything enclosed in square brackets ({}) is considered local to the function. When the JavaScript engine reads the function, it declares the variables; when it ends, it will destroy the variables.

As you can see, when "console.log ()" the result of the invoked greeting function, we are able to access to the WebsiteName after the function was played. This gives us the "Hello Karma Career‚" channel that we were looking for. As already mentioned the console.log () of the variable that was declared in the function generates an error because it cannot be defined.

, the reason WebsiteName is undefined is because variables are created inside functions when called, and then destroyed when the terminal instruction is executed. Anything outside the function does not have access to things inside the function, unless it has a particular configuration.

global reach

This next field is pretty much a literal translation of the phrase. A global scope takes elements declared outside of a function and reserves them in a space where all scripts, methods, and functions can access and use them for their own logic.

What does the above code do if console.log () counts at the end of the code ? What do you hope will happen?

Let’s look at the code: .

  1. variable counter declared and started in the global environment
  2. Added a function declared in the global environment.
  3. Add is invoked.
  4. the counter variable declared and started in the locale.
  5. does the local counter increase by 1 ‚áê because local and not global?
  6. The counter is returned. The ends of the function.
  7. Add is called again
  8. Perform steps 4 through 6.
  9. Repeat steps 3 again 6.a
  10. console.log (counter), ‚áê what is returned?

Since the function ends whenever the counter is 1, our local counter variable is again declared and restarted at 0 each time the function is executed. Whatever happens, the counter will always stop at 1 locally.

If a function finds a variable within its scope , it does not look for the global scope of the variable - so the global variable never changes. Then our console.log () will generate 0 since our variable defined closest in the environment of this instruction is in the global environment.

Lexical scope

Lexical scope is one of the fundamental concepts of JavaScript. It is the idea that the creation of a function or a variable will be accessible to certain parts of the code and therefore inaccessible to other parts of the code. It all depends on where the declaration of each variable and function is.

Let’s take a look at this block of code:.

Here we have a set of nested functions. The init () function declares a variable called var1, declares a function called second, and calls second ().

When the compiler goes through this code for the first time, it examines at a high level what we have:

At this point we can’t see anything else inside the init () - we only know the function exists when our init () function is called, the compiler takes another high level look to what’s inside the function:

  1. var1.
  2. seconds ()
  3. second () calls

the init () doesn’t know anything about what’s going on inside of the seconds () block. It can only see what is there in its lexical environment - the surrounding state.

Each function nested resides d n a smaller container, such as a set of nested Russian matryoshkas (see top of the page for example if you are not sure what they are). The dolls on her know what is going on inside their container and what has already happened or has / read in the parent. The larger doll, for example, only knows that the next doll exists in its container. She doesn’t know any of the other dolls in the set, just what is in her lexical environment (her state) and what has already happened (the outer realm).

Actually we know two things:.

  1. Outside domain cannot see internal domain
  2. The internal scope has access to the external domain.

Since the outside cannot see what is going on inside, we can safely say that this is a one-way relationship. The interior can see and use the variables from the exterior, but the exterior cannot see the interior. This is called lexical finality .

The beauty of lexical scope is that the value of a variable is determined by its placement in the code. Functions first look for the meaning of a variable in its locale - if it can not find it, it goes to the function that defines that function. If it doesn’t find it there, it goes up the chain to the next defined function.

This is becoming a very important concept in JavaScript that will come up again and again as you learn more about the JavaScript framework and how they work. You can go from the outside, but you can never go " up" the other way. This is very important when it comes to the main subject in question: closing .


The definition of closure is very similar to that of a lexical scope. The main difference between the two is that closure is a higher order function and lexical scope is not. A higher-order function has one basic characteristic: it returns a function or uses a function as a parameter.

The closure is a function which can reach its lexical scope, even when this function is called later.

Both closure and lexical scope have their own variables, can access variables and parameters of a parent function, and can use global variables. with the following code:

  1. Greeting () the function exists, but we do not know the contents yet.
  2. greetUser exists, but I don’t know its contents yet
  3. greetUser () - this line calls the previous one, which in turn calls the greeting () function.
  4. userName declare
  5. welcomeGreeting () exists, but I don’t know the content yet
  6. The Return statement under the block welcomeGreeting () returns the same function
  7. console.log (’Hello,’ + username) ; Our console.log here can access the parent scope to get the value of userName
  8. Declaration of the terminal which terminates the function and destroys the meaning of the variables in the code block.
  9. In this code, we pass information by nesting functions together so that we can access the parents’ litter later


    In this article, we’ve covered a fairly heavy JavaScript topic: scope and closures. I would recommend branching out and reading several articles on the subject. The way this is taught can come from a variety of perspectives - which means, of course, that there are many ways to learn. I hope you found this alphabet book useful! Good luck with your continued study on closures !

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    Javascript Closure Example find: Questions

    Finding the index of an item in a list

    5 answers

    Given a list ["foo", "bar", "baz"] and an item in the list "bar", how do I get its index (1) in Python?


    Answer #1

    >>> ["foo", "bar", "baz"].index("bar")

    Reference: Data Structures > More on Lists

    Caveats follow

    Note that while this is perhaps the cleanest way to answer the question as asked, index is a rather weak component of the list API, and I can"t remember the last time I used it in anger. It"s been pointed out to me in the comments that because this answer is heavily referenced, it should be made more complete. Some caveats about list.index follow. It is probably worth initially taking a look at the documentation for it:

    list.index(x[, start[, end]])

    Return zero-based index in the list of the first item whose value is equal to x. Raises a ValueError if there is no such item.

    The optional arguments start and end are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The returned index is computed relative to the beginning of the full sequence rather than the start argument.

    Linear time-complexity in list length

    An index call checks every element of the list in order, until it finds a match. If your list is long, and you don"t know roughly where in the list it occurs, this search could become a bottleneck. In that case, you should consider a different data structure. Note that if you know roughly where to find the match, you can give index a hint. For instance, in this snippet, l.index(999_999, 999_990, 1_000_000) is roughly five orders of magnitude faster than straight l.index(999_999), because the former only has to search 10 entries, while the latter searches a million:

    >>> import timeit
    >>> timeit.timeit("l.index(999_999)", setup="l = list(range(0, 1_000_000))", number=1000)
    >>> timeit.timeit("l.index(999_999, 999_990, 1_000_000)", setup="l = list(range(0, 1_000_000))", number=1000)

    Only returns the index of the first match to its argument

    A call to index searches through the list in order until it finds a match, and stops there. If you expect to need indices of more matches, you should use a list comprehension, or generator expression.

    >>> [1, 1].index(1)
    >>> [i for i, e in enumerate([1, 2, 1]) if e == 1]
    [0, 2]
    >>> g = (i for i, e in enumerate([1, 2, 1]) if e == 1)
    >>> next(g)
    >>> next(g)

    Most places where I once would have used index, I now use a list comprehension or generator expression because they"re more generalizable. So if you"re considering reaching for index, take a look at these excellent Python features.

    Throws if element not present in list

    A call to index results in a ValueError if the item"s not present.

    >>> [1, 1].index(2)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: 2 is not in list

    If the item might not be present in the list, you should either

    1. Check for it first with item in my_list (clean, readable approach), or
    2. Wrap the index call in a try/except block which catches ValueError (probably faster, at least when the list to search is long, and the item is usually present.)


    Answer #2

    One thing that is really helpful in learning Python is to use the interactive help function:

    >>> help(["foo", "bar", "baz"])
    Help on list object:
    class list(object)
     |  index(...)
     |      L.index(value, [start, [stop]]) -> integer -- return first index of value

    which will often lead you to the method you are looking for.


    Answer #3

    The majority of answers explain how to find a single index, but their methods do not return multiple indexes if the item is in the list multiple times. Use enumerate():

    for i, j in enumerate(["foo", "bar", "baz"]):
        if j == "bar":

    The index() function only returns the first occurrence, while enumerate() returns all occurrences.

    As a list comprehension:

    [i for i, j in enumerate(["foo", "bar", "baz"]) if j == "bar"]

    Here"s also another small solution with itertools.count() (which is pretty much the same approach as enumerate):

    from itertools import izip as zip, count # izip for maximum efficiency
    [i for i, j in zip(count(), ["foo", "bar", "baz"]) if j == "bar"]

    This is more efficient for larger lists than using enumerate():

    $ python -m timeit -s "from itertools import izip as zip, count" "[i for i, j in zip(count(), ["foo", "bar", "baz"]*500) if j == "bar"]"
    10000 loops, best of 3: 174 usec per loop
    $ python -m timeit "[i for i, j in enumerate(["foo", "bar", "baz"]*500) if j == "bar"]"
    10000 loops, best of 3: 196 usec per loop

    JSON datetime between Python and JavaScript

    4 answers

    kevin By kevin

    I want to send a datetime.datetime object in serialized form from Python using JSON and de-serialize in JavaScript using JSON. What is the best way to do this?


    Answer #1

    You can add the "default" parameter to json.dumps to handle this:

    date_handler = lambda obj: (
        if isinstance(obj, (datetime.datetime,
        else None
    json.dumps(, default=date_handler)

    Which is ISO 8601 format.

    A more comprehensive default handler function:

    def handler(obj):
        if hasattr(obj, "isoformat"):
            return obj.isoformat()
        elif isinstance(obj, ...):
            return ...
            raise TypeError, "Object of type %s with value of %s is not JSON serializable" % (type(obj), repr(obj))

    Update: Added output of type as well as value.
    Update: Also handle date

    What blocks Ruby, Python to get Javascript V8 speed?

    4 answers

    Are there any Ruby / Python features that are blocking implementation of optimizations (e.g. inline caching) V8 engine has?

    Python is co-developed by Google guys so it shouldn"t be blocked by software patents.

    Or this is rather matter of resources put into the V8 project by Google.


    Answer #1

    What blocks Ruby, Python to get Javascript V8 speed?


    Well, okay: money. (And time, people, resources, but if you have money, you can buy those.)

    V8 has a team of brilliant, highly-specialized, highly-experienced (and thus highly-paid) engineers working on it, that have decades of experience (I"m talking individually – collectively it"s more like centuries) in creating high-performance execution engines for dynamic OO languages. They are basically the same people who also created the Sun HotSpot JVM (among many others).

    Lars Bak, the lead developer, has been literally working on VMs for 25 years (and all of those VMs have lead up to V8), which is basically his entire (professional) life. Some of the people writing Ruby VMs aren"t even 25 years old.

    Are there any Ruby / Python features that are blocking implementation of optimizations (e.g. inline caching) V8 engine has?

    Given that at least IronRuby, JRuby, MagLev, MacRuby and Rubinius have either monomorphic (IronRuby) or polymorphic inline caching, the answer is obviously no.

    Modern Ruby implementations already do a great deal of optimizations. For example, for certain operations, Rubinius"s Hash class is faster than YARV"s. Now, this doesn"t sound terribly exciting until you realize that Rubinius"s Hash class is implemented in 100% pure Ruby, while YARV"s is implemented in 100% hand-optimized C.

    So, at least in some cases, Rubinius can generate better code than GCC!

    Or this is rather matter of resources put into the V8 project by Google.

    Yes. Not just Google. The lineage of V8"s source code is 25 years old now. The people who are working on V8 also created the Self VM (to this day one of the fastest dynamic OO language execution engines ever created), the Animorphic Smalltalk VM (to this day one of the fastest Smalltalk execution engines ever created), the HotSpot JVM (the fastest JVM ever created, probably the fastest VM period) and OOVM (one of the most efficient Smalltalk VMs ever created).

    In fact, Lars Bak, the lead developer of V8, worked on every single one of those, plus a few others.

    Django Template Variables and Javascript

    4 answers

    When I render a page using the Django template renderer, I can pass in a dictionary variable containing various values to manipulate them in the page using {{ myVar }}.

    Is there a way to access the same variable in Javascript (perhaps using the DOM, I don"t know how Django makes the variables accessible)? I want to be able to lookup details using an AJAX lookup based on the values contained in the variables passed in.


    Answer #1

    The {{variable}} is substituted directly into the HTML. Do a view source; it isn"t a "variable" or anything like it. It"s just rendered text.

    Having said that, you can put this kind of substitution into your JavaScript.

    <script type="text/javascript"> 
       var a = "{{someDjangoVariable}}";

    This gives you "dynamic" javascript.

    We hope this article has helped you to resolve the problem. Apart from Javascript Closure Example, check other find-related topics.

    Want to excel in Python? See our review of the best Python online courses 2022. If you are interested in Data Science, check also how to learn programming in R.

    By the way, this material is also available in other languages:

    Carlo Williams

    Berlin | 2022-12-10

    Thanks for explaining! I was stuck with Javascript Closure Example for some hours, finally got it done 🤗. I am just not quite sure it is the best method

    Walter Jackson

    Munchen | 2022-12-10

    Maybe there are another answers? What Javascript Closure Example exactly means?. Will use it in my bachelor thesis

    Dmitry Zelotti

    San Francisco | 2022-12-10

    Maybe there are another answers? What Javascript Closure Example exactly means?. I am just not quite sure it is the best method


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