Var Vs Let Javascript

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New features have been added to the JavaScript language with the release of ECMAScript6 (ES6). One of these characteristics affects the way we declare variables. With the addition of let and const, there were all kinds of questions about when to use each of the variable declarations.

In this article, we talk about each of the options, regarding their definition and use in the JavaScript environment.

Pre-ES6: Var

Prior to the release of ES6, the keyword used for variables was declaration of var .

Variables can also be redeclared and updated without errors

The first problem with var is obvious: redeclarations can cause bugs in your code if a variable has already been given the same name. There can be unintended consequences if something is declared again and you did not realize that the variable name has already been used in the codebase. Along with this, other peculiarities also arise regarding the scope and lifting.


The scope tells us what variables are available for us to use - we have variables that can be globally or locally scoped. What if we have the same variable declaration, but is global and the other is local?

When our var is declared outside the function, it is globally available to us globally. When "hello" is declared in the function, it is available only for the function - with functional or local scope

When we have a variable declaration which is the same, but the scope is different, the var in the function does not change the assignment of the global var, even with the invocation of helloWorld () . Since the var is declared in the function with the name "hello", it does not appear to be within the global reach of the alternate definition.


When a variable is hoisted in JavaScript, the declaration of functions and variables is moved to the start of their scope before code execution. The variable is declared but not initialized, so the initial value of the variable is undefined.

Here, "hello" is hoisted and declared at the top of your reach. So basically the computer interprets the code like this:

The JavaScript engine sees that hello exists but does not know how hello is defined until the next step.

Eventually the developers who helped create the ECMAScript standard realized that there were some minor issues with var that could cost them time and money. So they decided to create more stringent keywords. This led to the creation of "leave" and "cost" in ES6

ES6:. Let

The let keyword is very similar to the var keyword in many ways. The main differences are the way errors are returned and the scope of each keyword is defined.

Variables declared and initialized with let keyword can be reassigned, but they cannot be redeclared

When you run the above code in a JavaScript console, the last line returns an error:

SyntaxError: The identifier ’hello’ has already been declared

Unlike var, declaring variables using let will not allow a declaration to the same. Variable name. You can, however, reassign if you wish, as shown in the example above.


Champ-wise, let is very similar to var. In addition to the rules that the global or local scope gives us, there is an additional constraint with let. Variables declared in a code block are only available for that code block. This is similar to functional scope, as functions are blocks of code that can be carried even further by having separate blocks within that function.

There are no errors in this code because the two instances of hello are treated as separate variables because they have different scopes.


Hoisting is another area where the let and var declarations are similar. "Var", when raised, is initialized as undefined. However, "leave" generates a reference error if you try to use it before it is initialized.

Note that the let keyword is the best choice in terms of syntax. This is because errors are generated when you can try to redeclare a variable or use it before the initialization process.

ES6: Const

The const keyword has more stringent guide lines than the let keyword < / em>. With const, variables cannot be declared or reassigned. A TypeError is generated if you try to reassign to a const.

That said, when working with objects or arrays, the properties of the object or array may be updated. As long as the basic structure of the object or table does not change, you can always update it.

As the example shows, reassigning the task to a new set of properties results in a TypeError: Assignment to constant variable. error. Const cannot be reassigned or redeclared, except in cases where you update the unique property in an object

Scope and lift

Just like the let keyword, const is scope block and it does not initialize not when hoisted, so it will throw an error if you try to use it before initialization.

The const keyword is ideal to use until you can ’t & rsquo; not the use more This is when you need to transfer or put to update your variable


In this article, we have looked at the differences between let, var and cost . Of all the keywords we currently have, it’s best not to use var unless necessary. Instead, use let or const.

Var Vs Let Javascript cos: Questions


How do I install pip on macOS or OS X?

5 answers

I spent most of the day yesterday searching for a clear answer for installing pip (package manager for Python). I can"t find a good solution.

How do I install it?


Answer #1

UPDATE (Jan 2019):

easy_install has been deprecated. Please use instead.

Old answer:

easy_install pip

If you need admin privileges to run this, try:

sudo easy_install pip


Answer #2

⚡️ TL;DR — One line solution.

All you have to do is:

sudo easy_install pip

2019: ⚠️easy_install has been deprecated. Check Method #2 below for preferred installation!


⚡️ OK, I read the solutions given above, but here"s an EASY solution to install pip.

MacOS comes with Python installed. But to make sure that you have Python installed open the terminal and run the following command.

python --version

If this command returns a version number that means Python exists. Which also means that you already have access to easy_install considering you are using macOS/OSX.

ℹ️ Now, all you have to do is run the following command.

sudo easy_install pip

After that, pip will be installed and you"ll be able to use it for installing other packages.

Let me know if you have any problems installing pip this way.


P.S. I ended up blogging a post about it. QuickTip: How Do I Install pip on macOS or OS X?

✅ UPDATE (Jan 2019): METHOD #2: Two line solution —

easy_install has been deprecated. Please use instead.

First of all download the get-pip file

curl -o

Now run this file to install pip


That should do it.

Another gif you said? Here ya go!


Answer #3

You can install it through Homebrew on OS X. Why would you install Python with Homebrew?

The version of Python that ships with OS X is great for learning but it’s not good for development. The version shipped with OS X may be out of date from the official current Python release, which is considered the stable production version. (source)

Homebrew is something of a package manager for OS X. Find more details on the Homebrew page. Once Homebrew is installed, run the following to install the latest Python, Pip & Setuptools:

brew install python

Var Vs Let Javascript JavaScript: Questions

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.


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